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THE  SCIENCE  AND 
PRACTICE  OF  PHOTOGRAPHY 


Digitized  by  the  Internet  Archive 
in  2016 


https://archive.org/details/sciencepracticeo00roeb_0 


AUTOCIIROArE  PHOTOGRAPH 


THE  SCIENCE  AND 
PRACTICE  OF  PHOTOGRAPHY 


AN  ELEMENTARY  TEXTBOOK  ON  THE 
SCIENTIFIC  THEORY  AND  A 
LABORATORY  MANUAL 


BY 

JOHN  R.  ROEBUCK,  Ph.D, 

ASSISTANT  PROFESSOR  OP  PHYSICS  IN  THE 
UNIVERSITY  OP  WISCONSIN 


ILLUSTRATED 


D.  APPLETON  AND  COMPANY 

NEW  YORK  LONDON 


1920 


COPYRIGHT,  1918,  BY 
D.  APPLETON  AND  COMPANY 


Printed  in  the  United  States  of  America 


CH 


PREFACE 


Many  students  are  attracted  by  a course  in  photography 
who  have  had  little  acquaintance  with  science  and  less 
understanding  of  its  spirit.  There  is  no  reason  why  the 
methods  of  modern  science,  as  well  as  its  attitude,  cannot 
be  taught  by  a course  in  photography  as  well  as  by  a course 
in  quantitative  chemistry  or  in  the  theory  of  electricity.  It 
has  the  enormous  advantage  over  many  other  subjects  of 
starting  with  a vigorous  curiosity  on  the  part  of  the  student, 
and  of  maintaining  his  interest  by  the  strong  desire  to 
master  the  subject  for  his  own  use.  In  all  the  author’s 
experience  there  has  been  no  opening  for  criticism  of  the 
amount  of  work  his  students  were  willing  to  do;  the  diffi- 
culty has  been  to  keep  the  laboratory  closed  at  unreasonable 
hours. 

The  methods  of  science  are  necessarily  quantitative  where 
such  methods  can  be  applied.  The  quantitative  study  of 
the  dry  plate  offers  a very  attractive  example  to  induce  the 
student  to  chew  with  some  relish  the  tough  grist  of  close 
thinking.  For  the  student  who  has  already  developed  some 
powers  of  mastication,  the  case  is  not  particularly  difficult, 
and  its  understanding  is  absolutely  necessary  for  an  intelli- 
gent use  of  any  of  the  photographic  processes.  No  collec- 
tion of  apparently  empirical  rules  can  take  its  place.  It  was 
this  attitude,  deterrpining  the  treatment,  which  persuaded 
the  author  to  undertake  this  text.  Also  the  available  books 
offered  nothing  at  all  suitable  for  a laboratory  manual 
where  even  a small  number  of  students  were  to  be  cared 
for.  In  consequence,  the  broad  treatment  as  well  as  the 


V 


yi 


PREFACE 


details  are  the  product  of  experimental  selection  in  this 
laboratory.  The  course  has  now  been  given  for  seven  years, 
during  which  time  both  text  and  laboratory  manual  have 
been  rewritten  twice  in  mimeograph  form,  besides  receiving 
innumerable  additions  and  corrections.  The  author  hopes 
that  this  summary  of  his  experience  will  be  found  of  value 
to  other  instructors  and  students,  as  well  as  to  many  not  in 
university  work  yet  who  are  interested  in  the  real  under- 
standing of  their  subject. 

While  the  primary  purpose  of  the  book  is  a class-room 
text,  the  author  has  aimed  to  make  it  useful  to  many  earnest 
amateurs  by  collecting  and  systematizing  for  them  material 
now  widely  scattered  through  the  literature  of  photography. 
For  the  aid  of  such  workers  and  for  the  instructor  many 
references  are  given  to  the  literature  which  will  draw  atten- 
tion to  and  aid  in  the  study  of  the  original  work. 

In  the  British  Journal  Almanac  for  1916  (at  page  503) 
will  be  found  a list  of  the  journals  of  the  world  which 
print  photographic  information.  For  an  English-speaking 
worker  the  most  generally  useful  are:  the  British  Journal 
of  Photography  (U.  S.  agent,  George  Murphy,  59  E.  9th 
Street,  New  York),  the  Journal  of  the  Royal  Photographic 
Society  (Harrison  and  Sons,  45  Pall  Mall,  London,  S.  W.), 
the  Photo  Miniature  (Tennant  and  Ward,  New  York), 
American  Photography  (221  Columbus  Avenue,  Boston, 
Mass.),  and  possibly  still  more  popular  ones,  as  the  Photo 
Era  and  the  Camera. 

Any  teaching  in  science  should  keep  before  the  student 
that  he  is  just  beginning  and  that  the  most  experienced 
worker  carries  in  his  head  but  a small  proportion  of  the 
known  facts  and  even  of  the  accepted  generalizations  relat- 
ing to  his  subject.  The  literature  is  the  record  of  all  this 
information,  and  the  sooner  a student  can  be  brought  to 
turn  to  it  for  chance  reading  as  well  as  for  information  on 
specific  problems,  the  more  satisfactory  will  his  progress 


PREFACE 


vii 


be.  It  IS  not  easy  to  get  him  interested  to  the  extent  that 
he  will  read  voluntarily,  and  it  will  aid  materially  if  a few 
current  numbers  of  the  scientific  as  well  as  the  more  pop- 
ular journals  are  kept  in  the  laboratory,  where  they  will  be 
seen  thus  announcing  their  existence  and  be  picked  up  dur- 
ing waiting  moments.  For  the  library  it  hardly  needs  say- 
ing that  the  more  current  journals  in  all  languages  that  are 
kept,  and  the  more  complete  the  files  of  back  numbers,  the 
better  for  the  real  worker. 

To  some  not  directly  in  touch  with  research  it  may  be 
advisable  to  point  out  some  justification  for  burdening  the 
practical  photographer  with  so  much  work  in  mathematics, 
in  physics,  and  in  chemistry.  There  is  no  doubt  that  many 
photographers  doing  excellent  technical  work  know  very 
little  of  these  subjects  whose  application  is  the  basis  of  the 
methods  they  use.  This  last  statement  contains  the  justifi- 
cation, and  if  an  experimental  proof  is  desired,  it  is  only 
necessary  to  point  out  the  painfully  slow  progress  of  the 
methods  of  time  and  factorial  development  in  ousting  the 
old-time  guess  work  with  the  tray.  Further,  if  there  were 
an  intelligent  demand  from  the  user,  manufacturers  would 
soon  mark  their  plates  with  the  actual  development  speeds 
and  the  actual  sensitiveness.  Hardly  anyone  doubts  that 
the  empirical  worker  could  have  acquired  his  skill  with  less 
experience,  and  could  probably  improve  it  now,  by  under- 
standing the  relationships  among  the  variables  with  which 
he  works. 

The  course  as  outlined  in  this  book  requires  an  ele- 
mentary knowledge  of  mathematics,  of  physics,  and  of 
chemistry.  Good  high-school  algebra  as  well  as  good  high- 
school  courses  in  physics  and  in  chemistry  will  be  found 
sufficient  to  enable  the  student  to  take  the  further  steps 
required. 

No  extended  description  of  the  common  photographic 
apparatus  is  included  in  the  text  since  it  can  be  given  much 


viii 


PREFACE 


better  by  the  instructor  with  the  apparatus  beside  him.  The 
student  finally  becomes  familiar  with  it  by  use.  Any  dis- 
cussion of  the  variations,  with  uses  and  advantages,  would 
extend  the  text  unprofitably. 

The  discussion  of  lenses  has  to  stop  short  of  any  real 
mathematical  treatment  for  which  the  average  undergrad- 
uate has  no  preparation.  There  are  many  excellent  special 
treatises  now  to  be  had.  The  careful  testing  of  lenses 
requires  an  expensive  optical  outfit,  some  advanced  mathe- 
matics, and  considerable  practice.  In  any  case  the  main 
problems  of  the  young  photographer  are  not  with  his  lens 
performance,  and  very  few  expert  photographers  are  com- 
petent to  pass  upon  the  optical  questions  involved ; hence  it 
has  seemed  profitable  to  limit  the  discussion  of  lenses  to 
practically  that  contained  in  a good  book  on  college  physics. 

The  number  of  photographic  reproductions  in  the  text 
has  been  limited  by  consideration  of  the  fact  that  they  are 
reproductions  varying  necessarily  in  character  from  the 
originals,  and  by  the  fact  that  negatives  and  positives  them- 
selves should  be  displayed  in  the  laboratory  and  the  instruc- 
tor should  call  attention  to  the  points  in  the  student's  own 
work. 

My  thanks  are  due  to  Mr.  R.  C.  Williamson,  who  so  ably 
assisted  me  for  three  years  in  giving  this  course,  and  to 
whose  criticism  and  suggestion  many  improvements  in  both 
laboratory  course  and  text  are  due. 

John  R.  Roebuck. 


Physics  Laboratory 

University  of  Wisconsin 
1917 


CONTENTS 


PART  I 

GENERAL  THEORY 

CHAPTER  PAGE 

I.  HISTORICAL  DEVELOPMENT 


Art. 

I. 

The  Beauty  and  Utility  of  an 

Image 

3 

2. 

Niepce  .... 

4 

3- 

Daguerre  .... 

5 

4- 

Talbot  .... 

6 

5- 

Wet  Collodion 

7 

6. 

Collodion  Emulsions  . 

9 

7- 

The  Development  of  the  Gelatine  Dry  Plate 

10 

8. 

Gelatine  .... 

II 

9* 

Silver  Haloids 

12 

10. 

Dry  Plate  Making  . 

13 

II. 

Ammonia  Modification  . 

16 

12. 

Centrifuge 

16 

13- 

Processes  in  General  . 

17 

14. 

Summary  .... 

18 

[lOPERTIES  OF  THE  GELATINE 

DRY 

PLATE— EXPOSURE  AND  DEVEL- 

OPMENT 

15- 

The  Problem 

• 

• • 

19 

16. 

Hurter  and  Driffield  . 

• 

« • 

20 

17- 

Law  of  Absorption  of  Light 

• 

• # 

21 

18. 

Photometer  .... 

• 

• • 

23 

19. 

Fog 

25 

20. 

Ferrous  Oxalate 

• 

• • 

27 

21. 

Growth  of  Density 

• 

• • 

28 

22. 

Constant  Density  Ratios 

• 

• • 

29 

23- 

Application  in  Practice  . 

• 

• • 

31 

24. 

Intensification 

• 

• • 

32 

25- 

Characteristic  Curve  . 

• 

33 

26. 

Period  of  Correct  Exposure 

• 

37 

27- 

Perfect  Negative  . 

• 

39 

X 

CHAPTER 


CONTENTS 


PAGE 


28. 

Period  of  Underexposure 

40 

29. 

Period  of  Overexposure 

42 

30- 

Period  of  Reversal  .... 

43 

31- 

Dependence  of  Slope  on  Development  . 

43 

32. 

Development  factor  .... 

45 

33- 

Practical  Control  of  Amount  of  De- 

velopment  (Contrast) 

47 

33a. 

Constant  Time  at  Const.  Temperature 

47 

33b- 

Time  Adjusted  to  Temperature  . 

48 

33c. 

The  Watkins  Factorial  System  . 

49 

34. 

Contrast 

50 

35- 

Thickness  of  Film 

52 

36. 

Like  Negatives  from  Different  Exposures 

(Latitude) 

53 

37. 

Organic  Dry  Plate  Developers  . 

55 

38- 

The  Choice  of  a Developer  . 

56 

39- 

Estimation  of  Exposure  .... 

57 

40. 

Shutters 

58 

41. 

The  Stop 

61 

42. 

Dry  Plate  Speeds  .... 

64 

43- 

Preliminary  Exposure  .... 

67 

44. 

Character  of  Subject  .... 

68 

45- 

Lighting 

69 

PROPERTIES  OF  THE  GELATINE  DRY 
PLATE—COLOR  SENSITIVENESS 

46. 

Colored  Object  with  Ordinary  Plate  . 

72 

47- 

Color  Sensitiveness  Curves 

73 

48. 

Dyes 

74 

49- 

Absorption  Curves 

77 

50- 

Absorption  and  Sensitizing 

78 

SI* 

Practical  Application 

80 

52. 

Screening  Action 

83 

53- 

Color  Screens  ...... 

84 

54. 

Rendering  Color  Contrast  ...  * 

87 

LATENT  IMAGE  THEORIES 

55- 

Basic  Facts 

89 

56. 

Theories  . . ^ 

91 

57. 

Physical  Modification  .... 

91 

58. 

Silver  Grain 

92 

59- 

Silver  Sub-bromide  .... 

93 

60. 

Solid  Solution 

94 

61. 

Conclusion 

97 

CONTENTS 


xi 

CHAPTER  PAGE 

V.  NEGATIVE  DEFECTS 

62.  Classification  ......  99 

63.  Thin  Negatives 99 

64.  Intensification loi 

65.  Overexposure 103 

66.  Overdevelopment 104 

67.  Fog 104 

68.  Reduction 107 

69.  Local  Reduction 109 

70.  Conclusions 109 

71.  Frilling no 

72.  Dust Ill 

73.  Air  Bells 112 

74.  Black  Spots 1 12 

75.  Finger  Marks 113 

76.  Pressure  Marks 113 

77.  Halation 114 

78.  “Oyster  Shell  Marking”  . . . .118 

79.  Stain  . . . . ' , . . . . 118 

80.  Bubbles .118 

81.  Drying  Troubles  . . . . . .119 

VI.  POSITIVE  PROCESSES 

82.  Transparencies 121 

83.  Lantern  Slides 121 

84.  Blue  Prints 124 

85.  Printing  Out  Paper  (P.  O.  P.)  . . . 125 

86.  Developing  Paper 127 

87.  Sulphide  Toning 132 

88.  Platinotype 133 

89.  Carbon  Printing 135 

90.  Photo-engraving 139 

VII.  LENSES 

91.  Pinhole  Images 142 

92.  A Lens 145 

93.  Images 146 

94.  Size  of  Image  . . . . . . 149 

95.  Defects  in  Images  Formed  by  Lenses — 

Aberrations .150 

96.  Spherical  Aberration  . , . .151 

97.  Coma 152 

98.  Chromatic  Aberration  . . . .153 

99.  Achromatism 154 

100.  Curvature  of  Fiel . d . . . .159 


Xll 

CHAPTER 


CONTENTS 


PAGE 


lOI. 

Astigmatism 

* 

• 

• 

• 

159 

102. 

Distortion 

• 

• 

• 

• 

160 

103. 

Flare 

• 

• 

• 

• 

162 

104. 

Unequal  Illumination 

• 

• 

• 

• 

163 

105. 

Depth  of  Focus  . • 

■ 

9 

• 

• 

165 

106. 

Speed  of  Lenses  . 

• 

§ 

• 

• 

167 

107. 

Lens  Types  . . • 

9 

• 

168 

108. 

Telephoto  Lens 

• 

!s 

• 

170 

3L0R  PHOTOGRAPHY 

109. 

Classification  of  Methods 

• 

• 

• 

• 

173 

no. 

Lippmann  Process 

• 

• 

• 

174 

III. 

Pigment  Processes 

• 

• 

• 

• 

178 

1 12. 

Three  Negatives  . 

• 

• 

• 

• 

179 

113- 

The  Additive  Method 

• 

• 

• 

• 

179 

1 14. 

The  Subtractive  Method 

• 

• 

• 

180 

115. 

Carbon  Process  . 

• 

• 

• 

183 

1 16. 

Pinatype  Process  . 

• 

• 

• 

• 

183 

1 17. 

Traube  . 

• 

• 

• 

• 

185 

1 18. 

Photo-engraving  . 

• 

• 

• 

186 

119. 

Mosaic  Screen  Process 

• 

• 

• 

186 

120. 

Autochrome 

• 

• 

• 

• 

187 

121. 

Mosaic  Screen  Making 

• 

• 

• 

• 

190 

122. 

Conclusion  . 

• 

• 

• 

• 

193 

123. 

Bleach-out  Process 

• 

• 

• 

• 

193 

OOD  PICTURES 

124. 

Definition  . 

195 

125. 

Pictures 

196 

126. 

Picture  Composition  . 

198 

127. 

Unity 

• 

199 

128. 

Simplicity 

• 

200 

129. 

Foreground 

• 

201 

130. 

Sky-line  . 

• 

201 

131- 

Sky  .... 

• 

202 

132. 

Aerial  Perspective  . 

• 

203 

133- 

Color  in  Subject 

• 

204 

134. 

Lighting  . 

« 

206 

135- 

Notan 

• 

207 

136. 

Balance  . 

• 

208 

i37‘ 

Focal  Length  of  Lens  . 

• 

208 

138. 

Distortion  from  Short  Focal 

Length 

• 

210 

139. 

Angle  of  View  . 

• 

210 

140. 

Focusing 

211 

141. 

Distortion  from  Inclined  Plate 

• 

213 

CONTENTS 

xiii 

PAGE 

142. 

Lens  Axis 

. 213 

143* 

Exposure  and  Development 

. 214 

144. 

Printing 

. 214 

145. 

Mounting 

. 216 

146. 

Conclusion 

. 217 

APPENDIX 

DEVELOPMENT 219 

WATKINS  SPEED  LIST 220 

TABLE  OF  PLATE  SPEED  NUMBERS  . . .225 


PART  II 

LABORATORY  MANUAL 


PREFACE  . 

229 

GENERAL  DIRECTIONS 

EXPERIMENTS 

• 

• 

231 

I. 

Negatives  .... 

• 

• . 

234 

2. 

Negatives  Repeated  . 

. 

. . 

235 

3* 

Solution  Making  . 

. 

• . 

236 

4- 

Prints  on  Developing  Paper 

, , 

237 

5- 

To  Test  Systematically  for 

Correct  Ex- 

posure  .... 

• 

• • 

239 

6. 

Factorial  Development 

• 

• • 

241 

7- 

Contact  Lantern  Slides 

242 

8. 

Lantern  Slides  by  Reduction 

245 

9- 

Halation  .... 

246 

10. 

Intensification 

247 

II. 

Reduction  .... 

* 

251 

12. 

Color-sensitiveness 

252 

13- 

Spectrum  Photography  for 

Plate 

Color- 

Sensitiveness  . 

253 

14. 

Dyeing  and  Testing  of  Plates  . 

« , 

256 

15. 

Microphotography 

257 

16. 

Blue  Print  Paper 

258 

17. 

Gelatino-chloride  Paper  also  called  Print- 

ing  Out  Paper  (P.  0.  P.) 

, , 

260 

18. 

Enlarging  .... 

, 

. . 

262 

19. 

Sulphide  Toning  (sepia)  . 

. 

264 

20. 

Wet  Collodion  Plates  . 

. 

. 

265 

21.  Pictures  Showing  Clear  Distance  and 

Clouds 269 


XIV 


CONTENTS 


EXPERIMENTS 

22.  Autochromes 

23.  Carbon  Printing  . 

24.  Characteristic  Curve  . 
Finishing  Up  Work  . 

APPENDICES 


PAGE 

. 269 
. 271 
• 273 
. 274 


Laboratory  Apparatus 
Special  Apparatus 
Supplies  per  Student  . 

Chemicals  per  Student 
Photometers  . . . . 

Order  Slip 

Photography  Laboratory  Record 


276 

277 

278 


279 

280 
288 
288 


INDEX 


. 289 


PARTI 


GENERAL  THEORY 


CHAPTER  I 


HISTORICAL  DEVELOPMENT 

I.  The  beauty  and  utility  of  an  image  formed  by 
a lens  or  curved  mirror  inspires  every  observer  with 
the  desire  to  make  it  permanent.  The  direct  method 
for  accomplishing  this  is  to  have  the  light  forming 
the  image  act  upon  some  substance  in  such  a way 
as  to  leave  a record.  The  history  ^ of  photography 
begins  probably  with  the  observation  of  the  action 
of  light  on  substances,  and  a great  variety  of  such 
observations  are  recorded.  In  particular  the  Swed- 
ish chemist,  Scheele,  studied  the  action  of  light  on 
silver  chloride,  proving  the  decomposition  of  the 
substance,  thus 

AgCl  » Ag+Cl 

Along  with  such  observations  one  must  place  also 
studies  on  lenses  and  on  the  camera  obscura.  The 
idea  of  the  latter  is  attributed  to  an  Italian  in  the 
sixteenth  century.  A certain  amount  of  chemical 

* For  more  detail  in  the  history,  see  W.  J.  Harrison,  ^‘History 
of  Photography." 


3 


4 


PHOTOGRAPHY 


knowledge  was  essential  to  progress  in  photo- 
graphy, particularly  the  chemistry  of  the  salts  of 
silver  which  have  always  occupied  the  leading  place 
among  the  substances  affected  by  light.  Toward 
the  end  of  the  eighteenth  century  chlorine,  bro- 
mine, and  iodine  had  all  been  discovered  and  studied 
so  that  Sir  Humphry  Davy  and  Thomas  Wedg- 
wood were  able  to  experiment  more  intelligently 
than  their  predecessors.  They  coated  plates  with 
the  salts  of  silver,  silver  nitrate,  silver  chloride, 
silver  bromide,  and  silver  iodide,  and,  exposing  them 
to  light,  made  pictures.  They  discovered  that  the 
action  of  light  differed  somewhat  with  the  differ- 
ent substances  used  and  with  the  color  of  the  light 
allowed  to  fall  upon  them.  But  they  could  not  make 
their  pictures  permanent,  as  they  knew  of  no  way 
to  remove  the  unacted  on  silver  salt,  so  that  expo- 
sure to  light  in  the  course  of  time  turned  it  all 
black. 

2.  Niepce. — Following  this  a Frenchman, 

Niepce,  succeeded  in  making  pictures  by  using  the 
fact  that  light  renders  bitumen  insoluble  in  certain 
oils  (probably  on  account  of  the  oxidation  of  the 
bitumen — compare  the  present  day  carbon  process). 
He  coated  a metallic  surface  with  a layer  of  the 
bitumen  and  exposed  it  to  light  in  a camera  some- 
times for  a whole  day,  and  then  with  the  oil  washed 
away  the  parts  still  soluble.  Since  all  the  insoluble 


historical  development 


5 


part  was  on  the  surface  and  the  remaining  soluble 
part  next  the  metal,  this  washing  removed  all  the 
fine  lines  (detail)  where  the  insolubility  did  not 
extend  right  through  to  the  metal.  Nevertheless 
he  obtained  permanent  pictures  and  probably  the 
first  ones. 

3.  Daguerre. — Later  Niepce  and  Daguerre 
worked  together  in  partnership  but  it  was  not  till 
after  the  former's  death  that  Daguerre  succeeded 
in  making  permanent  pictures.  Neglecting  varia- 
tions during  the  process  of  discovery,  the  final  proc- 
ess consisted  in  making  a polished  silver  plate,  ex- 
posing it  to  iodine  vapors,  thus  forming  silver  iodide 
on  the  surface.  This  plate  was  then  exposed  in 
the  camera,  and  later  developed  by  exposing  to  the 
vapor  of  mercury,  this  process  of  development  being 
discovered  by  Daguerre  quite  by  accident  but  fol- 
lowed up  with  great  skill.  The  picture  was  then 
fixed,  early  in  the  work  by  boiling  in  strong  sodium 
chloride  solution,  but  later,  when  the  solvent  action 
of  sodium  hyposulphite  solution  on  the  salts  of 
silver  was  pointed  out  to  him,  he  changed  over  to 
it.  The  compound  itself  as  well  as  its  action  on  the 
salts  of  silver  had  only  been  discovered  a few  years 
before.  The  pictures  were  very  fine,  full  of  detail, 
and  more  permanent  than  most  of  our  prints  now. 
It  quickly  leaped  into  popularity  and  won  a pension 
for  Daguerre  from  the  French  government.  Good 


6 


PHOTOGRAPHY 


examples  may  still  be  seen.  The  exposure  was  of 
the  order  of  five  minutes,  but  this  was  lessened 
later  by  using  a mixture  of  the  haloids,  chlorine, 
bromine,  and  iodine,  instead  of  iodine  alone,  to  act 
on  the  silver  plate.  This  process  held  the  field  for 
about  15  years,  when  it  was  displaced  by  the  collo- 
dion process. 

4.  Talbot. — At  this  same  time,  1840-1850,  Tal- 
bot in  England  worked  out  a process  which  was  in 
some  respects  superior  to  Daguerre's  though  not  on 
the  whole.  Paper  was  brushed  over  with  silver 
nitrate  solution  and  dried.  It  was  then  wet  in 
potassium  iodide  solution  and  made  still  more  sen- 
sitive to  the  action  of  light  by  soaking  in  a gallic 
acid  solution — this  really  acting  also  as  a devel- 
oper. He  fixed  them  at  first  by  soaking  in  potas- 
sium bromide  solution  but  later  used  hyposulphite 
solution.  This  gave  a negative,  and  by  waxing  or 
oiling  the  paper,  it  was  possible  to  lay  it  on  another 
piece  of  paper  similarly  treated  and  by  exposing  to 
light  make  a print  which  was  a positive.  The  proc- 
ess allowed  of  duplication  and  was  cheaper  than 
Daguerre's,  but  the  latter  were  much  finer  in  detail 
and  more  permanent,  so  that  the  Daguerreotypes 
held  the  popular  fancy,  and  Talbot's  process  was 
never  generally  used. 

During  these  years  Herschel  suggested  the  use 
of  glass  for  support  of  the  sensitive  substance,  and 


HISTORICAL  DEVELOPMENT 


7 


a second  Niepce  actually  covered  a plate  with  albu- 
men to  hold  the  silver  salt,  but  the  exposure  was 
long, — 5-10  minutes,  and  the  process  did  not  attract 
much  attention. 

5.  Wet  Collodion. — The  foundation  for  the  col- 
lodion process  ^ was  laid  when  the  Swiss  chemist, 
Schonbein,  discovered  the  action  of  nitric  and  sul- 
phuric acids  on  ordinary  cotton.  Acting  for  a long 
time  with  very  concentrated  acid  makes  guncotton 
an  explosive,  but  if  for  a shorter  time  or  in  more 
dilute  solution  it  forms  a substance  called  soluble 
pyroxyline.  A solution  of  this  in  a mixture  of  alco- 
hol and  ether  is  called  collodion,  and  is  still  used 
in  surgery  for  covering  up  cuts  and  abrasions,  as  it 
dries  to  a strong  film.  The  soluble  pyroxyline  mixed 
with  some  camphor  makes  celluloid.  Scot  Archer 
applied  these  discoveries  of  soluble  pyroxyline  in  the 
collodion  process,  which,  as  it  came  from  his  hands, 
quickly  displaced  all  previous  processes.  It  held 
the  field  till  about  1880  and  is  still  used  in  many  of 
the  photo-engraving  establishments.  Either  the 
soluble  pyroxyline  or  its  solution  called  collodion 
can  now  be  bought  from  photographic  supply 
houses,  but  for  many  years  the  photographer  had 
to  make  his  own  pyroxyline  by  soaking  cotton  in  a 

* For  an  excellent  detailed  description  of  the  modern  process, 
see  Foxlee,  “Wet  Collodion  Process,”  British  Journal,  54,  483 

(1907). 


8 


PHOTOGRAPHY 


mixture  of  equal  parts  of  nitric  and  sulphuric  acids 
and  washing  thoroughly.  The  product  was  dis- 
solved in  a mixture  of  alcohol  and  ether,  a little 
soluble  iodide  and  bromide  added,  and  a film  formed 
on  clean  glass  by  pouring  the  solution  onto  the  glass 
plate  so  as  to  cover  it  all,  and  letting  the  excess 
drain  off  while  the  rapidly  drying  material  left  a 
coating  on  the  glass  surface.  As  soon  as  the  film 
hardened  it  was  dipped  in  a solution  of  silver  ni- 
trate, thus  precipitating  silver  bromide  and  silver 
iodide  in  the  film.  It  was  exposed  while  wet  and 
developed  while  yet  wet  by  a ferrous  sulphate  solu- 
tion, which  reduced  to  metallic  silver  the  silver 
haloid  which  has  been  acted  on  by  light.  The  plate 
was  fixed  by  flooding  it  with  a solution  of  sodium 
hyposulphite,  in  strong  solution,  which  dissolves  out 
the  unacted  on  silver  haloid.  Many  workers  pre- 
ferred a solution  of  potassium  cyanide  for  this  pur- 
pose but  it  is  exceedingly  poisonous.  The  plate  was 
finally  washed  in  water.  These  reactions  may  be 
represented  in  chemical  symbols  thus, 

AgNOa+KBr  =KN03+AgBr 
and  AgN03+KI  =KN03+AgI 

and  in  development 


FeS04+AgBr  = Ag+Fe(S04)Br 


HISTORICAL  DEVELOPMENT 


9 


On  attempting  to  fix  in  weak  hypo  solution 

AgBr+Na2S203=NaBr+AgNaS20s  (insoluble)  , 

and  in  strong  hypo  solution 

2AgBr+3Na2S203  = 2 NaBr-hAg2Na4 (8203)3  (soluble) 

and  in  potassium  cyanide  solution 

AgBr+2KCN=KBr+KAg(CN)2  (soluble) 

This  process  is  a difficult  one  on  account  of  the 
great  care  required,  but  it  gives  beautiful  negatives, 
very  clear  with  abundant  detail,  and  is  still  unex- 
celled for  lantern  slides.  For  the  enthusiast  it  has 
the  advantage  that  the  whole  process  is  entirely  in 
his  own  hands  and  also  for  this  reason  gives  the 
clearest  laboratory  insight  to  the  photographic  proc- 
ess. We  will  spend  a short  time  on  it  in  the  labora- 
tory. (See  Exp.  20.) 

6.  Collodion  Emulsions. — On  account  of  the 
difficulty  of  handling  the  plates  wet,  persistent  ef- 
forts were  made  to  perfect  some  way  to  use  them 
dry,  but  the  sensitiveness  to  light  was  always  much 
reduced,  though  a goodly  number  of  ways  were 
worked  out  as  possible.  The  best  of  these  modifi- 
cations, which  might  have  held  the  field  but  for 
the  gelatine  plate,  was  Bolton's  '‘Washed  Collodion 
Process."  ^ The  collodion  with  soluble  bromide  in 
it,  not  iodide,  was  mixed  with  silver  nitrate  dis- 

* See  also  article  on  Orthochromatic  Collodion  Emulsions  in 
British  Journal,  55,  418  (1908). 


lO 


PHOTOGRAPHY 


solved  in  alcohol.  The  resulting  liquid  is  called  an 
emulsion  on  account  of  its  resemblance  to  a well- 
shaken  mixture  of  oil  and  water.  It  is  filled  with 
fine  particles  of  silver  bromide,  which  is  not  soluble 
in  the  alcohol-ether  mixture,  and  besides,  holds  in 
solution  the  other  products  of  the  reaction  as  well 
as  possibly  some  silver  nitrate  if  it  was.  present  in 
excess.  The  emulsion  was  then  dried  in  a thin  layer 
and  thoroughly  washed  to  remove  the  soluble  salts, 
dried  and  again  dissolved  in  the  alcohol-ether,  when 
it  was  ready  for  coating  the  plates,  which  were  used 
after  they  were  dry,  but  were  moistened  again  with 
silver  nitrate  solution  before  development.  Plates 
made  on  this  plan  were  on  the  market  for  some 
years.  The  great  difficulty  with  all  these  dried  col- 
lodion plates  is  that  the  collodion  dries  to  such  a 
hard  and  compact  film  that  it  does  not  absorb  the 
solutions  used  for  development. 

7.  The  development  of  the  gelatine  dry  plate 
soon  put  all  these  processes  out  of  use  except  for 
very  special  purposes.  A great  many  early  workers 
experimented  with  gelatine,  but  with  many  failures 
till  about  1880,  when  the  plates  were  perfected  suffi- 
ciently to  outclass  the  collodion  so  completely  as  to 
displace  it  in  three  or  four  years,  their  great  ad- 
vantages being  their  keeping  qualities,  portability, 
and  speed.  It  will  be  sufficient  for  our  purpose  to 
describe  one  method  for  making  gelatine  plates  and 


HISTORICAL  DEVELOPMENT  1 1 

not  to  attempt  to  trace  the  modifications  during  dis- 
covery. 

8.  Gelatine  itself  is  a very  interesting  substance.”* 
It  is  obtained  from  animal  tissue,  horns,  skin,  hoofs, 
etc.,  by  digesting  them  in  hot  water,  for  the  high- 
grade  gelatine  without  boiling,  but  for  the  lower 
grades  and  for  glue  even  heating  in  a closed  boiler 
under  pressure.  It  has  to  be  freed  as  completely 
as  possible  from  all  fat  and  also  from  all  inorganic 
salts.  Isinglass  is  made  from  the  air  bladder  of  a 
fish,  the  sturgeon,  in  much  the  same  way.  Gela- 
tine belongs  to  the  class  of  bodies  called  colloids; 
it  is  made  up  principally  of  two  substances,  both 
organic  and  nitrogenous,  called  glutin  and  chon- 
drin,  the  former  being  in  excess  in  photographic 
gelatines,  making  them  set  more  readily  and  melt 
at  a higher  temperature — that  is  ''hard’'  gelatine. 
Dried  in  the  air  it  still  retains  15-20%  of  water,  and 
if  put  in  water  it  swells  up,  absorbing  several  times 
its  own  volume  of  water,  becoming  very  soft  and 
mechanically  weak  but  not  fluid.  If  while  still 
swelled  the  temperature  be  raised,  depending  on  the 
gelatine  used,  to  30-40°  C.,  it  melts  to  a thick  liquid, 
and  on  cooling  it  hardens,  "sets,”  "gelatinizes,”  or 
"gels,”  again  but  slowly,  not  immediately  on  cool- 

* See  also  Abney,  “Photography  with  Emulsions,”  p.  102; 
Brothers,  “Manual  of  Photography,”  p.  257;  Burton  and  Prin- 
gle, “Processes  of  Pure  Photography,”  p.  51. 


12 


PHOTOGRAPHY 


ing.  In  the  dry  form  it  keeps  indefinitely  but  when 
wet  or  dissolved  in  water  it  quickly  putrifies,  as  it 
makes  an  excellent  medium  for  the  growth  of 
molds  and  bacteria,  and  it  then  generally  loses  its 
setting  power,  becoming  liquid  or  remaining  liquid. 
Boiling  in  water  steadily  lowers  the  melting  point 
till  finally  the  setting  power  is  destroyed.  It  ab- 
sorbs chlorine,  bromine  and  iodine  readily,  which  is 
important  in  its  photographic  use,  as  it  thus  allows 
the  more  complete  decomposition  of  the  silver  salt 
by  the  light,  the  removal  of  the  halogen  from  the 
field  of  action  preventing  it  from  recombining  with 
the  silver.  When  boiled  with  silver  nitrate  the  gela- 
tine solution  turns  red  and  the  silver  salt  is  decom- 
posed. Treatment  of  the  swelled  gelatine  with  quite 
a large  number  of  substances,  such  as  alum,  chrom- 
alum,  formalin,  etc.,  raises  the  melting  point  or 
even  renders  it  quite  insoluble  in  water.  It  is  not 
soluble  in  strong  alcohol,  and  is  precipitated  from 
water  solution  by  the  addition  of  alcohol;  even  the 
swelled  gelatine  when  immersed  in  alcohol  loses 
water  and  shrinks  down  to  a hard  film. 

g.  Silver  Haloids. — Of  the  compounds  of  the 
haloids,  chlorine,  bromine,  and  iodine,  with  silver, 
the  bromide  is  the  most  important  one  in  photo- 
graphic work.  Stas,^  a famous  Belgian  chemist, 
studied  the  different  forms  in  which  it  occurs  and 

® Stas,  Annal.  de  Chem,  et  Phys.,  5th  Ser.,  3,  289  (1874). 


HISTORICAL  DEVELOPMENT  13 

how  it  may  be  changed  from  one  to  the  other.  Pre- 
cipitated directly  from  water  solutions,  it  is  finely 
divided  and  a faint  yellow,  and  quickly  gathers  into 
larger  particles  and  settles  to  the  bottom.  If  boiled 
for  a long  time  in  water  out  of  the  light  the  particles 
subdivide  till  it  will  remain  suspended  for  a long 
time  in  water,  and  this  finely  divided  form  is  exceed- 
ingly sensitive  to  light.  It  may  be  made  also  by  pre- 
cipitating dilute  boiling  silver  nitrate  solution  with 
dilute  boiling  ammonium  bromide  solution.  When 
precipitated  from  a solution  containing  a small 
quantity  of  one  of  many  other  substances,  including 
gelatine,  the  particles  of  silver  bromide  carry  down 
with  them  some  of  the  foreign  material — and  this 
property  has  an  important  bearing  on  the  manufac- 
ture of  gelatine  emulsions. 

10.  Dry  Plate  Making. — ^As  an  example  of  the 
preparation  of  a gelatino-bromide  emulsion  the  fol- 
lowing from  Abney  ® will  be  described.  Materials 
and  their  quantities  are  as  follows : 

1.  Pot.  Iodide  o.^g  in  3.5  cc  water 

2.  Pot.  Bromide  S.yg  in  40  cc  water 

3.  Nelson’s  No.  i Photo  Gelatine  2g. 

4.  Silver  Nitrate  ii.4g  in  15  cc  water 

( Heinrich’s  Gelatine  log. 

\ Nelson’s  No.  i Gelatine  6g. 

The  presence  of  both  bromide  and  iodide  in- 
creases the  sensitiveness  over  that  with  either  alone. 


"Abney,  “Treatise  on  Photography.” 


H 


PHOTOGRAPHY 


The  use  of  the  two  kinds  of  gelatine  is  for  the  pur- 
pose of  making  the  film  hard  enough  when  wet  to 
stick  sufficiently  strongly  to  the  glass,  and  yet  of 
low  enough  melting  point  to  absorb  the  solutions 
readily.  Both  lots  of  gelatine  are  washed  with 
water  to  free  them  from  dust  and  then  allowed  to 
swell  in  water.  To  the  silver  nitrate  a drop  of 
hydrochloric  acid  is  added  to  make  the  emulsion 
acid.  No.  3 Gelatine  is  melted  by  standing  the 
drained  swelled  gelatine  in  a water  bath  at  50°  C., 
and  the  silver  nitrate  added  and  well  mixed. 
Then  No.  2 Pot.  Bromide  is  added  slowly  drop  by 
drop,  with  constant  agitation,  of  course  in  the  dark 
room,  till  ^ is  added,  when  No.  i Pot.  Io- 
dide is  mixed  with  the  remaining  Pot.  Bromide 
and  the  mixture  added  to  the  gelatine  as  before, 
drop  by  drop.  This  makes  the  precipitated  haloid 
very  finely  divided,  and  the  liquid  with  suspended 
precipitate  is  called  an  emulsion.  It  will  be  noted 
that  the  bromide  and  iodide  are  present  in  more  than 
sufficient  quantity  to  combine  with  all  the  silver 
nitrate,  so  that  there  is  no  free  silver  nitrate  left 
to  react  with  the  gelatine.  If  a little  of  this  emul- 
sion is  now  examined  by  looking  through  it,  it  will 
be  found  to  be  ruby  red.  Place  the  flask  in  boiling 
water  in  the  dark  for  40  minutes.  If  plates  be 
made  from  it  at  intervals,  it  will  be  found  that  the 
sensitiveness  will  be  increasing  steadily  and  the  sil- 


HISTORICAL  DEVELOPMENT  1 5 

ver  bromide  particles  increasing  in  size.  At  some 
time  the  sensitiveness  will  begin  to  decrease  and  the 
boiling  should  be  stopped  short  of  this  point.  It 
will  be  noted  that  part  of  the  gelatine  was  reserved 
so  that  it  would  not  be  affected  by  the  boiling.  It 
is  now  warmed  to  melting  and  mixed  with  the  emul- 
sion, and  the  whole  allowed  to  set. 

In  the  emulsion  besides  the  water,  gelatine  and 
silver  haloids,  there  will  be  the  following  substances, 
KBr,  KI,  KNO3,  HCI  (see  reactions  in  Art.  5), 
and  if  these  latter  arc  removed  the  plate  will  be 
much  more  sensitive.  To  this  end  the  jelly  is  now 
well  broken  up  by  squeezing  through  strong  cloth, 
and  washed  and  soaked  and  washed  for  hours.  It 
is  then  well  drained  and  is  ready  to  coat  the  plates 
as  soon  as  it  is  melted  by  warming.  If  a small  quan- 
tity of  alcohol  is  added  before  coating  the  plates  it 
will  increase  the  sensitiveness  somewhat. 

To  make  the  layer  of  gelatine  adhere  to  the  glass 
surface  through  all  the  later  operations  it  is  neces- 
sary that  the  glass  surface  be  absolutely  clean.  No 
one  who  has  not  actually  attempted  to  thoroughly 
clean  a surface  realizes  how  difficult  it  is.  All 
grease  can  be  removed  by  a hot  alkali  solution,  par- 
ticularly if  assisted  by  friction.  Hot  chromic  acid 
or  nitric  acid  will  destroy  all  other  organic  sub- 
stances and  dissolve  a good  many  inorganic.  Wash 
thoroughly,  finishing  with  distilled  water.  Many 


i6 


PHOTOGRAPHY 


workers  finish  by  polishing  with  French  chalk  (fine- 
ly divided  calcium  carbonate)  on  clean  chamois 
leather  or  cloth,  but  the  latter  must  be  clean  and 
must  not  be  touched  by  the  fingers  on  the  part  used 
for  polishing.  A layer  of  hardened  gelatine  or  of 
albumen  adheres  to  the  glass  very  strongly,  and  the 
emulsion  adheres  to  this  layer  strongly  also  so  that 
most  plates  on  the  market  are  made  this  way  when 
the  cleaning  does  not  need  to  be  quite  so  thorough. 

The  plates  are  placed  on  a level  surface  and  a 
measured  quantity  of  the  warm  liquid  emulsion 
poured  on  and  spread  to  the  corners  with  a glass 
rod  in  a very  dim  ruby  light.  The  emulsion  soon 
sets  when  it  is  best  placed  in  a current  of  dry  air 
in  the  dark  till  quite  dry. 

11.  Ammonia  Modification.— The  above  method 
is  known  technically  as  the  boiling  method,  where 
the  great  sensitiveness  is  largely  due  to  the  pro- 
longed heating  during  which  the  silver  bromide 
goes  into  the  sensitive  form.  Another  method 
reaching  about  the  same  sensitiveness,  is  to  add 
some  ammonia  at  the  point  above  when  the  boiling 
commenced,  and  to  then  let  it  stand  for  a day  or  so 
at  room  temperature.  Some  workers  use  one  meth- 
od, some  the  other.  Repeated  melting  and  solidify- 
ing of  the  finished  emulsion  also  increases  the  sensi- 
tiveness. 

12.  Centrifuge. — The  boiling  of  the  emulsion  as 


historical  development  17 

above,  deteriorates  the  gelatine  used  a great  deal, 
and  it  is  of  advantage  to  get  rid  of  this  spoiled  gela- 
tine. This  can  be  done  following  the  boiling  by 
whirling  the  liquid  in  a centrifugal  machine,  which 
is  a cylindrical  vessel  arranged  to  be  rotated  rapidly 
about  its  axis.  The  heavy  particles  of  silver  bro- 
mide are  by  this  means  driven  to  the  walls  of  the 
vessel  and  the  fluid  part  may  be  poured  out,  carry- 
ing with  it  practically  all  the  soluble  salts  and  most 
of  the  old  gelatine.  The  silver  bromide  is  then 
added  to  fresh  melted  gelatine,  and  when  thor- 
oughly mixed  is  ready  for  coating  the  plates.  Boil- 
ing the  emulsion  without  any  gelatine  present  gives 
but  a poorly  sensitive  plate. 

13*  So  far  we  have  been  concerned  with  the  im- 
portant outstanding  photographic  processes  only. 
There  are  a great  many  others  of  more  or  less  im- 
portance, with  other  light  sensitive  substances  and 
supports.  There  are  books  filled  with  short  terse 
descriptions  of  such  processes  literally  by  the  hun- 
dred. We  will  need  to  consider  a few  of  them  later 
when  we  come  to  discuss  methods  of  making  posi- 
tives. It  should  also  be  pointed  out  that  except  for 
some  other  difficulties  in  the  handling  of  the  flexible 
support,  the  films  for  film  cameras  are  made  like  the 
dry  plates  are  made.  For  some  reason  films  have 
not  been  nearly  as  fast  as  the  fastest  plates,  but  they 


i8 


PHOTOGRAPHY 


have  been  fast  enough  for  most  work.  Lately  a 
faster  film  has  been  put  on  the  market. 

14.  Summary. — To  sum  up  again  the  reactions 
involved  in  the  case  of  the  making  of  a negative 
from  a gelatine  dry  plate,  thus  in  the  light 

AgBr=Ag+Br  or  2AgBr=Ag2Br+Br 

and  we  will  leave  the  discussion  of  which  occurs 
and  of  the  theories  generally  till  more  of  the  facts 
are  in  hand.  Then  in  the  developer 

2Ag2Br+H20+R  = 2Ag2+2HBr+RO 

or  where  there  is  a particle  of  the  metallic  silver 
due  to  the  action  of  light,  in  the  silver  bromide 
particle, 

2AgBr+H20+R  = 2Ag+2HBr+RO 

where  ‘‘R”  is  the  developer  which  is  in  all  cases 
a material  which  will  take  up  oxygen.  The  func- 
tion of  the  carbonate  in  the  developer  is  partly  to 
keep  the  solution  from  getting  acid  from  the  forma- 
tion of  the  HBr.  Then  in  the  strong  sodium  hypo- 
sulphite (also  called  thiosulphate)  solution 

2AgBr+3Na2S20s  = 2NaBr4-Na4Ag2(S203)s 

and  the  complicated  compound  formed  is  soluble 
in  water. 


CHAPTER  II 


PROPERTIES  OF  THE  GELATINE  DRY  PLATE- 
EXPOSURE  AND  DEVELOPMENT 

15*  Now  that  we  have  the  description  of  the 
manufacture  of  gelatine  dry  plates,  and  have  used 
them  somewhat  in  the  laboratory,  the  next  step 
logically  is  the  investigation  of  the  properties  of 
the  plate.  Here  a qualitative  investigation  is  not  of 
much  use,  but  the  properties  must  be  measured  and 
the  relation  between  the  different  experimental 
quantities  expressed  in  mathematical  form.  For 
example  we  must  answer,  from  the  standpoint  of 
experiment,  such  questions  as. 

What  is  the  relation  between  the  amount  of 
exposure  and  the  density  of  the  negative? 

How  does  the  kind  of  developer  and  the  time 
of  development  affect  the  density  of  the  neg- 
ative, and  affect  the  relation  between  the 
densities  of  different  parts  of  the  negative? 

How  should  we  measure  and  how  express  the 
speed  of  different  plates? 

and  many  more  such  questions.  The  answers  are 
very  valuable  both  in  the  experimental  use  of  the 

19 


20 


PHOTOGRAPHY 


plates  and  the  interpretation  of  what  we  get  in  the 
negative.  In  1890  two  Englishmen,  Hurter  and 
Driffield  publishea  ^ a description  of  some  quantita- 
tive experimental  work  on  the  gelatine  dry  plate, 
which  was  the  first  of  many  papers  by  many  work- 
ers. It  was  a splendid  piece  of  work,  and  stirred 
up  a vigorous  controversy  which  has  hardly  died 
out  yet.  Their  results  have  been  checked  many 
times  and  in  different  ways,  and  are  now  accepted 
as  correct.  Since  they  answer  some  of  the  ques- 
tions which  it  is  worth  while  to  ask,  a little  time 
can  be  spent  profitably  over  their  paper. 

16.  Hurter  and  Driffield  begin  with  a definition 
of  a perfect  negative — ''when  the  amount  of  light 
transmitted  through  its  different  gradations  is  in 
inverse  ratio  to  that  which  the  corresponding  parts 
of  the  original  subject  sent  out.’’  It  is  to  be  care- 
fully noted  that  this  does  not  say  the  same  actual' 
differences  but  only  the  same  proportions ; the  actual 
differences  may  be  either  greater  or  less.  That  is 
if  one  subject  sends  out  twice  the  light  to  the  cam- 
era which  another  does,  then  it  should  be  repre- 
sented in  the  image  by  a place  of  twice  the  opacity. 

^ Hurter  & Driffield,  “Photochemical  Investigations/’  Jour,  of 
Soc.  Chem.  Ind.  9,  455  (1890)  ; Photo  Miniature,  No.  56,  where 
it  is  rewritten  by  Driffield;  Photo  Miniature,  No.  66,  applica- 
tion to  the  general  problem  of  development.  For  a very  com- 
plete treatment  and  many  references  see  Sheppard  & Mees, 
“Investigations  on  the  Theory  of  the  Photographic  Process,” 
Longmans,  Green  & Co.,  1907. 


GELATINE  DRY  PLATE 


21 


The  proportion  and  not  the  actual  difference  is  the 
important  matter. 

17.  Law  of  Absorption  of  Light. — The  next  step 
is  to  investigate  how  the  light  absorbed  by  a trans- 
lucent layer  depends  on  the  incident  light  and  on  the 
layer.  This  is  important  as  it  is  the  absorption  of 
light  by  the  sensitive  film  which  makes  possible  its 
development  into  a negative,  and  the  absorption  of 
light  by  the  negative  which  makes  printing  possible. 
Suppose  we  find  by  measurement  that  a certain  film 
will  absorb  yi  the  light  coming  up  to  it,  that  is  it 
transmits  If  we  double  the  light  coming  up  we 
will  find  that  the  fraction  transmitted  will  remain 
the  same,  although  the  film  is  both  absorbing  and 
transmitting  more  than  it  did  before.  More  meas- 
urements would  show  this  to  be  the  general  rule, 
that  the  fraction  transmitted  is  a constant  inde- 
pendent of  the  illumination.  In  algebraic  symbols 


where  I is  the  incident  and  i the  transmitted  light, 
and  i/m  is  the  fraction  transmitted,  where  m is  a 
constant  independent  of  I but  dependent  on  the  film 
used.  Suppose  now  that  two  such  layers  be  used, 
each  layer  will  transmit  ^ of  the  light  which  gets 
to  it,  but  the  second  layer  only  gets  ^ of  the  inci- 
dent light,  so  that  the  fraction  transmitted  by  the 
two  films  together  will  be  ^ X ^ of  the  originally 


22 


PHOTOGRAPHY 


incident  light.  For  3 layers  it  would  be  similarly 
( Evidently  if  the  fraction  absorbed  was  dif- 

ferent from  the  same  expression  would  still 
hold,  that  is  if  i/m  was  transmitted  by  the  first 
layer,  then  (i/my  would  be  transmitted  by  n lay- 
ers. That  is,  the  more  general  form  of  equation  i 
above  is 

j = (i/m)“  or  j = (m)“  (2) 

and  for  convenience  let  us  write  m = (10)’' which 
gives 

[ = ((lo)*)”  = (io)>“  = (io)«  (3) 

The  ratio  is  called  the  opacity  because  it  is  the 

amount  of  light  which  must  fall  upon  the  film  to 
have  unit  quantity  get  through.  The  power  kn, 
which  is  usually  written  d,  depends  only  on  the 
absorbing  body  and  is  called  its  density.  In 
these  terms  the  equation  3 may  be  written 

0=io^  (4)2 

and  this  equation  is  the  relation  we  set  out  to  deter- 
mine. 

2 For  the  student  who  has  studied  logarithms  this  may  be 
written  more  conveniently  thus: 

j = (i/m)“  = m-"  = 

whence  loge  i = kin  and  logio  i = — - — (kin)  = kn 
1 1 2.303 

or  logio  0 = kn  = d 


GELATINE  DRY  PLATE 


23 


18.  Photometer. — To  measure  the  density  of  a 
photographic  plate  it  is  possible  to  separate  the  sil- 
ver from  a measured  area  of  a negative,  and  from 
this  get  the  density.  But  this  method  is  very 
tedious  and  on  account  of  the  very  small  quantities 
of  silver,  is  not  accurate.  It  is  better  to  measure  the 
opacity  of  the  plate  iind  from  it  get  the  density  by  use 


of  equation  4 above.  For  this  purpose  Hurter  and 
Driffield  modified  the  ordinary  bench  photometer  to 
make  the  apparatus  pictured  in  the  diagram.  Fig.  i. 
Outside  of  the  box  at  each  end  is  placed  a lamp,  and 
the  central  box  can  be  moved  toward  either  end,  thus 
getting  more  light  from  one  lamp  and  less  from  the 
other.  In  addition  to  this  the  board  with  the  wedge- 
shaped  opening  can  be  moved  in  and  out,  so  chang- 
ing the  vertical  length  of  the  opening  in  that  end  of 
the  box,  and  so  varying  the  amount  of  light  admitted 


24 


PHOTOGRAPHY 


from  that  lamp.  The  position  of  each  can  be  read 
on  the  scale  attached.  The  arrangements  inside  the 
inner  box  is  shown  in  Fig.  2.  The  vertical  sep- 
tum in  the  center  of  this  inner  box  has  a thin  paper 
middle,  in  the  center  of  which  again  is  a spot  which 
has  been  made  more  transparent  with  grease.  The 
two  inclined  mirrors  in  the  back  of  this  box  enable 


Fig.  2, 


the  observer  to  see  both  sides  of  this  greased  paper 
at  once.  If  the  illumination  on  the  two  sides  of  the 
grease  spot  is  different,  the  spot  looks  lighter  or 
darker  than  the  surrounding  paper.  When  the  il- 
lumination on  the  two  sides  is  equal  the  spot  be- 
comes invisible,  and  then  the  strength  of  the  two 
sources  will  be  directly  as  the  squares  of  their  dis- 
tances from  the  grease  spot.  Then  one  light  aper- 
ture is  covered  with  the  plate  to  be  measured,  the 


GELATINE  DRY  PLATE 


25 


small  box  is  shifted  till  the  grease  spot  becomes  in- 
visible again;  from  these  two  settings  I/i  is  easily 
calculated,^  and  from  that  the  density.  To  deter- 
mine how  much  dependence  could  be  placed  on  the 
instrument,  they  determined  the  density  of  known 
solutions  of  India  ink,  and  the  determined  and 
known  values  agree  to  about  one  per  cent. 


Table  i 


India  Ink 
added  to 
100  cc  Water 

Density 

Density 

ealeulated 

measured 

see 

0.240 

0.240 

10 

0.480 

0.500 

15 

0.720 

0.750 

20 

0.960 

0.950 

25 

1 . 20 

1.245 

30 

1.44 

1.44 

35 

1.68 

1.665 

40 

1.92 

1.885 

This  is  consequently  all  that  should  be  expected  in 
its  regular  use.  The  table  showing  the  measure- 
ments is  given  above. 

19.  Fog.  — Obviously  if  one  wants  the  den- 
sity of  the  silver  forming  the  negative  image  it  will 
not  do  to  include  with  this  the  density  of  the  gela- 
tine and  of  the  glass  plate,  for  both  these  absorb 
some  light.  Also  there  is  always  some  chemical  fog, 
that  is  silver  reduced  which  is  not  due  to  the  action 
of  the  light  which  formed  the  negative.  To  get  the 

* See  appendix  on  photometer,  p.  280. 


26 


PHOTOGRAPHY 


density  due  entirely  to  the  action  of  the  light  it  is 
necessary  to  allow  for  these  other  densities,  and 
they  made  a practice  of  leaving  a small  part  of  the 
plate  unexposed,  measuring  the  light  absorbed  by  it, 
and  subtracting  this  amount  from  the  density  of  the 
rest  of  the  plate  to  get  the  density  due  to  light. 
For  example  to  test  the  eflfect  of  time  of  develop- 
ment, they  exposed  a plate  except  a narrow  strip 
along  one  edge,  then  cut  up  the  plate  at  right  angles 
to  this  unexposed  strip,  and  developed  each  of  these 
strips  a different  length  of  time.  The  following 
table  gives  the  actual  measurements. 


Table  2 


Time  of  development, 
minutes 

2.5 

5- 

7.5 

10. 

15- 

20. 

Density  exposed  plate  . . . 

.670 

• 965 

1-245 

1.420 

1-755 

1-945 

Density  unexposed  plate  . 

.200 

•345 

.415 

•505 

•575 

.710 

Density  due  to  light 

.470 

.620 

.830 

•915 

1. 180 

1-235 

Percent  developed 

38. 

50.2 

67.2 

74.1 

95-5 

100. 

Note  that  the  density  of  the  negative  grows  with 
time  of  development,  but  the  density  due  to  light 
reaches  a maximum  in  this  case  in  about  15  min- 
utes; also  note  what  a large  proportion  the  fog  is 
of  the  total  density,  and  that  it  continues  to  grow 
steadily  even  after  the  light  image  has  reached  its 
maximum.  The  first  fog  figure  (.200)  is  the  den- 
sity almost  exactly  of  the  glass  plate  and  the  gela- 
tine, and  this  should  be  subtracted  from  the  later  fog 


GELATINE  DRY  PLATE 


27 


readings  to  get  the  true  fog.  At  small  values  the 
fog  is  approximately  uniform  all  over  the  plate  and 
consequently  will  not  affect  the  printing  qualities 
except  to  require  longer  to  make  the  printing  ex- 
posure. 

20.  Ferrous  Oxalate. — All  the  alkaline  develop- 
ers have  this  undesirable  property  of  producing  a 
lot  of  fog,  which  is  especially  undesirable  in  this 
kind  of  work.  The  ferrous  oxalate  developer 
which  works  in  acid  solution  produces  very  little 
fog,  even  an  hour’s  development  of  an  unexposed 
plate  giving  hardly  enough  fog  to  measure.  The 
developer  is  made  by  mixing  strong  solutions  of 
ferrous  sulphate  and  potassium  oxalate.  These 
react  to  give  ferrous  oxalate,  thus: 

FeS04-}-K2C204  = FeC204 -{-1^2^04 

which  is  not  soluble  in  water  but  is  soluble  in  an 
excess  of  the  potassium  oxalate  solution.  When  in 
contact  with  exposed  silver  bromide,  they  react 
thus: 

K2C2O4-I-  2FeC204~l-  2AgBr  =Fe2(C204)3-l-  2KBr-|-  2Ag 

that  is  the  ferrous  salt  changes  over  to  ferric,  me- 
tallic silver  is  formed,  while  the  amount  of  acid  pres- 
ent does  not  change.  Unfortunately  the  solution 
absorbs  oxygen  from  the  air  fairly  rapidly,  and  the 
ferrous  salt  not  being  very  soluble,  soon  precipitates, 
but  does  little  harm  when  it  does  precipitate  and 


28 


PHOTOGRAPHY 


merely  calls  for  a little  care  in  not  using  the  devel- 
oper too  long.  On  account  of  its  non-fogging  prop- 
erty they  used  it  for  most  of  the  quantitative  work. 

21.  Growth  of  Density. — They  exposed  one-half 
the  plate  to  light  and  then  cut  it  up  so  as  to  include 
a part  unexposed  and  a part  exposed  on  each  piece, 
developed  in  the  oxalate  developer,  fixed  and  washed 
it.  After  drying  the  density  was  measured  in  the 
photometer,  subtracting  the  fog  to  get  the  density 
due  to  the  action  of  the  light.  The  first  step  was 
to  determine  the  way  in  which  the  density  grows 
in  the  developer.  For  this  purpose  different  pieces 
were  developed  for  different  lengths  of  time,  and 
with  developer  differing  in  the  amount  of  the  two 
constituents,  of  water  or  of  potassium  bromide. 
The  resulting  densities  are  given  in  the  following 
table : 


Table  3 


Time 

Mins. 

I 

II 

Density  exclusive  of  fog. 

Ill  IV  V Mean  Calculated 

5 

•365 

•350 



•215 

•332 

. 290 

10 

•525 

.460 



•305 

.462 

.464 

15 

.615 

•550 

•79S 

•570 

.410 

•569 

.568 

20 

.615 

•575 



.... 

.420 

.582 

.628 

25 

. 700 

.650 



.660 

.665 

30 

. 700 

.660 

.860 

.670 

•450 

• 645 

.687 

45 

I .000 

•715 

•515 

•715 

•713 

60 

.... 

.740 

.... 

.740 

.719 

This  table  shows  that,  as  in  the  previous  table, 
the  densities  grow  rapidly  at  first,  then  more  slowly, 
finally  reaching  a maximum  value.  The  rate  of 


GELATINE  DRY  PLATE 


29 


growth  is  quite  different  in  the  different  cases,  I-V, 
and  besides  depending  on  the  developer  depends  on 
the  gelatine  of  which  the  plate  is  made  and  on  the 
age  of  the  plate.  By  a method  of  guess  and  try  they 
were  led  to  the  equation 


d = D(i-a^)  (S) 

where  d is  the  density  at  the  time  t,  D is  the  final 
density  and  a is  a fraction  chosen  to  suit  the  case. 


\0 


j 

r 

/I 

r\i 

'31=  kit 

PklT 

Influence  ofUmc 

Curve  Calculated 
•Experirneotttl  Evidence/ 

/ 

' 

/ 

yrn 

/ 

10 


eo 


50 


Fig.  3. 


40  ^0 

MiNUTCS 


60 


This  equation  enabled  them  to  calculate  the  density 
at  any  time  during  the  progress  of  development. 
This  is  well  shown  by  the  agreement  of  the  column 
mean  and  calculated  of  Table  3,  this  data  is  plotted 
in  Fig.  3,  where  the  curve  is  the  calculated  points 
and  the  experimental  points  are  the  dots. 

22.  Constant  Density  Ratios. — This  formula  has 
some  very  interesting  consequences,  for  example 
when  t is  large,  a being  a fraction,  a^  becomes  small, 


30 


PHOTOGRAPHY 


and  when  t is  sufficiently  large,  a*  becomes  small 
enough  in  comparison  with  i to  be  neglected,  that 
is  d becomes  the  same  as  D.  That  is  to  say  the  for- 
mula shows  that  the  density  will  reach  a maximum 
which  is  in  agreement  with  experiment.  Or  sup- 
pose we  had  two  places  of  different  exposure  on  the 
same  plate  where  the  final  densities  will  be  Di  and 
D2,  then  the  densities  at  any  time  t will  be  given  by 


di  = Di  (i  — a*)  and  d2  = D2  (i  — a*) 


where  the  time  from  the  start  of  development  is  the 
same  in  both  cases,  namely  t,  and  a is  the  same  all 
over  the  same  plate.  If  we  divide  one  of  these 
equations  by  the  other,  the  bracketed  part  divides 
out,  and  we  have. 


d2  D2 


(Sa) 


That  is,  the  ratio  of  the  densities  at  any  time  t dur- 
ing development  is  the  ratio  of  the  final  densities, 
and  since  this  is  true  for  any  time  it  is  true  for  all 
time  during  development.  That  is,  the  ratio  of  the 
densities  at  the  two  places  is  always  the  same  no 
matter  when  you  stop  development.  This  does  not 
say  that  the  actual  densities  do  not  change,  they 
grow  greater  and  their  difference  grows  greater, 
that  is,  contrast  grows  greater  during  development. 
But  nevertheless  one  place  is  always  the  same  pro- 
portion denser  than  the  other.  They  confirmed  this 
result,  as  it  is  very  important,  by  a lot  of  experi- 


GELATINE  DRY  PLATE 


31 


ments,  using  plates  from  different  makers  and  many 
other  developers  than  the  oxalate.  Nor  did  chang- 
ing the  developer  during  the  progress  of  develop- 
ment affect  this  result. 

23.  Application  in  Practice. — It  follows  that  the 
ratios  are  independent  of  any  ordinary  treatment 
during  development.  Of  course  covering  only  part 
of  the  plate  with  the  developer  will  upset  this  rule, 
or  warming  it  locally,  for  example,  with  the  fingers. 
It  is  of  course  a series  of  densities  which  make 
a negative  and  it  is  the  ratios  between  them  and 
not  their  absolute  values  which  are  most  important 
in  the  negative  and  in  the  print.  It  is  fortunate 
that  it  is  not  a matter  of  special  skill  and  manipu- 
lation to  keep  the  ratios  true  to  the  light  values 
making  the  negative  and  therefore  true  to  the  sub- 
ject. A correctly  exposed  negative  fills  their  defi- 
nition of  a perfect  negative  almost  entirely  inde- 
pendent of  the  process  of  development.  All  that 
can  be  accomplished  by  ordinary  manipulation  dur- 
ing development  is  to  fix  the  density  at  one  point  in 
the  negative  and  hence  to  fix  it  at  all  other  points. 

Any  attempt  by  manipulation  during  develop- 
ment to  bring  out  one  set  of  detail  and  to  subordi- 
nate another  is  useless.  Moreover  if  the  plate  was 
not  correctly  exposed  it  can  only  be  very  imper- 
fectly remedied  during  development.  All  there  is 
any  need  to  do  is  to  interrupt  the  progress  of  devel- 


32 


PHOTOGRAPHY 


opment  when  the  highest  and  lowest  densities  are 
not  too  far  apart  in  actual  value  for  the  print  to 
show  their  difference,  nor  so  close  together  that 
they  do  not  use  a good  deal  of  the  range  of  densi- 
ties of  the  print.  It  is  for  this  reason  that  you  have 
been  urged  to  control  the  development  of  your  plates 
by  following  the  time  for  which  they  are  in  the  de- 
veloper. By  choosing  the  time  of  development  to 
suit  the  developer  used  and  the  brand  of  plate,  and 
then  making  the  exposure  such  as  will  give  a suit- 
able density  for  printing,  one  will  get  the  best  pic- 
tures. But  where  one  guesses  at  the  exposure  and 
then  tries  to  adjust  the  treatment  in  the  developer 
so  as  to  correct  errors  of  exposure,  there  will  be 
two  things  to  blame  the  failures  on,  and  no  real 
guide  as  to  which  one  was  actually  wrong.  But 
by  choosing  and  adhering  to  a suitable  time  of  de- 
velopment, one  will  be  sure  that  the  errors  are  in 
the  exposure,  and  so  by  watching  the  successes  and 
failures  learn  to  make  correct  exposures.  In  fact 
tank  development  is  the  logical  thing. 

24.  Intensification — The  next  experiment  was 
to  test  the  effect  of  intensification,  and  the  same  rule 
of  constant  density  ratios  was  found  here.  The 
increase  is  proportional  to  the  density  before  in- 
tensification. An  example  is  given  in  Table 
4.  The  plate  was  exposed,  developed  with  fer- 
rous oxalate,  the  densities  measured,  and  after- 


GELATINE  DRY  PLATE 


33 

wards  intensified  and  the  densities  measured  again. 


Table  4 


Expo- 

Before Intensific. 

After  Intensific. 

sure 

Density  Di  Ratio 

Density  D2  Ratio 

D1/D2 

10. 0 

•31 

1 .0 

. 60 

1 .0 

1-93 

14,0 

•50 

1 .61 

.91 

1-5 

1 . 82 

20.5 

.67 

2 . 16 

1.30 

2. 16 

1.94 

29-3 

.86 

2.77 

1. 71 

2.85 

1 . 98 

41.9 

1-03 

3-32 

2.15 

3-5 

2.08 

60.0 

1.30 

4.19 

2.56 

4.2 

Mean 

1 . 96 
I-9S 

It  is  to  be  noted  that  the  two  columns  of  ''ratios’^ 
are  similar  line  for  line,  and  also  the  factor  by 
which  the  density  has  been  changed  by  intensifica- 
tion is  the  same  for  all  the  different  densities,  with- 
in the  error  of  experiment.  Both  facts  show  that 
the  density  ratios  are  not  changed  by  this  method 
of  intensification,  which  is  then  merely  another  way 
of  continuing  the  process  of  development.  Many 
other  intensifiers,  however,  do  change  the  ratios, 
(Art.  64),  as  also  do  most  reducers  (Art.  68). 

25.  Characteristic  Curve. — Having  answered 
these  questions  with  respect  to  developers  and  de- 
velopment, the  next  step  was  to  investigate  the 
effect  of  exposure  on  the  production  of  density.  For 
this  purpose  it  is  advisable  to  choose  some  unit  in 
which  to  express  the  exposure,  and  they  used  the 
light  from  a standard  candle  burning  for  one  sec- 
ond at  one  meter  distant  from  the  plate.  This  they 


34 


PHOTOGRAPHY 


call  the  candle-meter-second,  and  it  has  been  very 
generally  used  since  their  time.  Using  this  unit  to 
measure  the  exposure  given,  they  gave  a series  of 
exposures  increasing  by  factors  of  two  to  different 
parts  of  a plate,  by  uncovering  one  particular  part 
of  the  plate  at  one  time;  and  developed  it  in  ferrous 
oxalate. 


Table  5 


Exposure  Density  Difference 


Exposure  Density  Difference 


0.625 

.045 

80 

I . no 

•255 

1-25 

•055 

0.010 

160 

1 . 270 

. 260 

2.50 

.085 

0.030 

320 

1-555 

.285 

5.00 

•175 

0.090 

640 

1.885 

•330 

10. 0 

.250 

0.075 

1280 

2.088 

.203 

20.0 

.460 

0.210 

2560 

2.262 

.174 

40.0 

•755 

0.295 

5120 

2.352 

.090 

It  will  be  seen 

that  every  time 

the  exposure  ; 

doubled  the  density  increases,  at  first  slowly,  then 
considerably  and  (disregarding  errors  of  experi- 
ment) from  40  C.  M.  S.  exposure  up  to  1280,  every 
time  the  exposure  is  doubled  nearly  an  equal  addi- 
tion of  density  is  the  result,  the  addition  to  the 
density  being  on  an  average  0.266,  but  after  an  ex- 


posure of  1280  further  doubling  produces  less  and 
less  increase  in  density.  The  first  few  densities  are 
too  small  to  admit  of  accurate  measuring. 

This  series  of  results  is  represented  graph- 
ically in  Fig.  4,  exposures  horizontally  and  densi- 
ties vertically;  from  this  diagram  it  will  be  seen  at 


GELATINE  DRY  PLATE 


35 


once  how  rapidly  densities  grow  at  first  as  exposure 
is  increased,  and  how  slowly  at  last  the  densities 
tend  toward  a limit. 


/ 

-V 

r 

1 

f/  1000  eooo  3000  4000  ^ooo 

CkPOSORC  . CANDLe-MCtRE-SeC0ND5 

Fig.  4. 


There  is  something  more  to  be  learned  from  a 
series  of  exposures  running  to  still  higher  values, 
and  they  give  another  experiment  very  similar  to 
the  last. 

Table  6 


Exposiure  Density  Difference 


I 

.060 

• • • • 

2 

. 160 

. 100 

4 

.340 

. 180 

8 

,500 

. 160 

16 

•715 

• 215 

32 

.940 

.225 

64 

I -345 

• 40s 

128 

1-875 

• 530 

256 

2.290 

• 4IS 

512 

2-535 

•245 

Exposure  Density  Difference 


1024 

2.985 

0.450 

2048 

3-115 

0.130 

4096 

3.280 

0.165 

8192 

3-405 

0.125 

16384 

3-508 

+0.103 

32768 

3-474 

-0.034 

65536 

3.280 

-0.194 

131072 

3.128 

— 0.162 

262144 

2.920 

— 0.208 

524288 

2.464 

-0.456 

36 


PHOTOGRAPHY 


Note  that  toward  the  end  of  this  series  of  ex- 
posures the  addition  of  more  exposure  actually  les- 
sens the  density  on  development.  To  represent  this 
diagrammatically  as  in  the  previous  case  would  re- 
quire a diagram  impracticably  large  and  would  not 
give  information  of  any  value.  But  if  the  method 
of  plotting  be  changed  it  can  be  made  into  by  all 
means  the  best  way  of  treating  this  data.  The 
change  required  is  simple,  instead  of  giving  equal 
distances  horizontally  to  equal  amounts  of  expo- 
sure, give  equal  distances  to  equal  multiples  of 
exposure.  That  is  in  the  case  here  each  multiplica- 
tion by  2 moves  the  point  an  equal  distance  to  the 
right.  The  result  of  this  is  to  crowd  closer  to- 
gether relatively  the  points  at  the  long  exposure  end 
of  the  list.  This  will  give  us  the  curve.  Fig.  5, 
which  is  called  the  Characteristic  Curve.^ 

The  curve  consists  of  four  distinct  branches. 
The  first  curved  portion,  convex  downward,  is  called 
the  period  of  underexposure.  The  straight  middle 
portion,  the  most  important  part,  is  called  the  period 
of  correct  exposure.  The  part  following  this  and 
bending  to  the  right,  concave  downward,  including 
up  to  the  maximum  density,  is  called  the  period  of 

The  student  who  has  studied  logarithms  will  notice  that 
we  are  here  plotting  the  logarithms  of  the  exposure  on  the 
horizontal  axis  while  plotting  the  densities  linearly.  And 
therefore  for  the  straight  line  portion  of  the  curve  (log.  E — 
log.  i)  is  proportional  to  the  density. 


GELATINE  DRY  PLATE 


37 


overexposure.  And  fourth,  the  final  bending  down 
part  is  called  the  period  of  reversal.  We  will  con- 
sider the  second  part  in  detail  first. 


Fig.  5. 

26.  Period  of  Correct  Exposure.' — In  the  dia- 
gram, ab  is  a straight  line  within  the  error  of  ex- 
periment. Let  X be  any  point  on  it,  and  draw  xy 
at  right  angles  to  cy.  Then  wherever  x may  be  on 
ab,  the  triangle  xyz  remains  similar,  that  is 

y (yz)  = xy 

where  y (gamma)  keeps  the  same  value,  being  the 
number  of  times  that  one  side  is  greater  than  the 
other. 


PHOTOGRAPHY 


38 

Let  us  examine  further  the  way  we  have  plotted 
the  exposure.  If  E is  the  exposure,  we  may  put 

E « 10^ 


where  v is  a suitable  number  and  then  multiplying 
E by  a factor  means  making  an  addition  to  V.  For 
example  if  we  multiply  by  10,  then 

loE  »=  lo^Xio  = 

Or  if  we  multiply  by  any  number  X,  we  can  always 
write 

X = 10“ 

where  n is  a suitable  number,  and  therefore 
XE  = 10^X10“  * io^+“ 

So  that  we  really  plot  the  index  v of  some  suitable 
number  such  as  10.  In  the  actual  case  above  plotted 


GELATINE  DRY  PLATE 


39 


the  suitable  number  was  2 and  when  we  had  chosen  a 
distance  to  represent  unit  exposure,  we  plotted  the 
index  in  the  series  1,2^  2^  2^  2^,  etc. 

If  we  have  been  careful  to  make  our  plotting  of 
|he  above  diagram  suitably,  we  may  write 

E = 

and  let  us  choose  ^'i’’  a number  such  that 


i = 10®" 


Dividing  one  by  the  other,  we  get 


E 

i ""  10"" 


jQ(cy-cz) 


10^ 


But  in  the  discussion  above  it  is  shown  that 

zy  = xy/y  = d/y 
Whence  E/i  = 10 

27.  Perfect  Negative.- — Experiment  shows  that 
as  development  proceeds  the  curve  ab  swings  up 
about  the  point  z as  center,  that  is,  becoming  stead- 
ily steeper.  Let  the  amount  of  development  be  so 
chosen  that  the  y in  our  above  expressions  is  unity, 
that  is,  the  slope  of  the  line  ab  will  have  to  be  45°. 
For  this  special  case,  then, 

E/i  = 10® 


It  was  shown  above  in  Art.  17  that 


O - 10^ 


40  PHOTOGRAPHY 

and  therefore 

E/i  = O (6)5 

For  any  particular  curve  the  value  of  i is  a con- 
stant, and  therefore  the  opacity  (O)  is  directly 
proportional  to  the  exposure  (E).  The  amount  of 
light  transmitted  is  inversely  proportional  to  the 
opacity  so  that  this  statement  becomes,  the  amount 
of  light  transmitted  is  inversely  proportional  to  the 
exposure,  which  is  the  definition.  Art.  i6,  that  Hur- 
ter  and  Driffield  give  of  a perfect  negative. 

Hence  it  is  possible  to  make  such  a perfect  nega- 
tive by  filling  two  conditions:  (i)  the  exposure  of 
every  portion  of  the  plate  must  be  such  as  to  bring 
it  onto  the  straight  line  portion  of  the  characteristic 
curve,  and  (2)  the  time  of  development  must  be 
such  that  y will  be  unity. 

28.  Period  of  Underexposure. — This  includes 
the  initial  bend  of  the  characteristic  curve  lying  be- 

“ For  the  more  advanced  student  this  may  be  more  conven- 
iently written  thus: 

d 

tan  a = =y 

log  E — log  i 

where  a is  the  angle  between  the  straight  part  and  the  E axis- 
For  the  case  of  7 = i,  d = log-y 

But  d = log  O (Art.  19)  .*.  O —~ 

and  since  i is  a constant  for  all  the  straight  part 

O : E 

or  the  opacity  is  proportional  to  the  exposure. 


GELATINE  DRY  PLATE 


41 


tween  the  letters  c and  a in  Fig.  6.  The  slope 
of  any  part  here  is  always  less  than  that  of  the  part 
ab,  so  that  for  the  same  time  of  development  the 
densities  will  be  much  less,  and  also  the  density  dif- 
ferences between  any  two  points,  that  is,  the  con- 
trast will  be  less,  and  the  negative  will  in  general 
be  thin  and  flat.  Not  only  that  but  the  contrasts 
will  not  be  graded  as  they  are  in  the  subject,  but 
will  be  relatively  greater  than  they  should  be  in  the 
longer  exposed  parts,  and  detail  will  be  lacking  in 
the  shadows.  The  print  will  not  be  true  to  the  sub- 
ject. 

Note  carefully  that  the  above  is  true  for  the  same 
time  of  development  or  in  better  words  for  the 
same  development  factor  (gamma).  Many  texts 
and  workers  say  that  ''underexposure  gives  con- 
trasty negatives,’’  which  is  not  strictly  the  case.  It 
should  read  that  "the  longer  development  usually 
given  underexposures  leads  to  contrasty  negatives.” 
When  the  progress  of  development  of  a negative  is 
followed  by  looking  at  it  or  through  it,  the  watcher 
stops  development  when  the  negative  has  reached 
the  opaqueness  to  which  he  is  accustomed.  The 
result  is  that  the  short  exposure  negative  is  devel- 
oped a long  time,  and  as  contrast — density  differ- 
ence— grows  with  development,  this  results  in  a 
contrasty  negative.  On  the  other  hand  a liberally 
exposed  negative  grows  black  quickly  all  over  in 


42 


PHOTOGRAPHY 


the  developer  and  is  removed  before  the  contrast 
has  had  time  to  grow  great,  resulting  in  a soft  or 
even  flat  negative.  For  the  same  subject  and  light- 
ing the  difference  in  the  contrasts  in  the  two  cases 
is  produced  in  the  developer,  not  in  the  exposure. 

Exposures  falling  on  the  part  of  the  curve  ca 
are  so  small  as  hardly  to  give  printing  density  even 
on  prolonged  development.  Such  a negative  may 
be  intensified,  so  continuing  the  process  of  develop- 
ment, and  thus  be  built  up  to  good  printing  density. 
The  contrast  will  however  become  very  great,  giv- 
ing hard,  black  and  white  prints  which  are  quite 
untrue  to  the  subject. 

There  are  many  negatives  which  for  the  greater 
part  of  the  subject  are  satisfactorily  exposed  but 
for  some  dark  objects  or  weakly  lighted  ones  are 
given  such  exposures  as  to  fall  on  this  initial  part  of 
the  curve.  In  the  print  these  objects  do  not  appear 
as  they  should  but  are  weak  or  even  lacking  in 
detail.  The  old  maxim  of  ''expose  for  the  shadows 
and  the  high-lights  will  take  care  of  themselves’’  is 
worth  remembering. 

29.  Period  of  Overexposure. — This  includes  the 
upper  curved  portion  as  far  as  the  highest  point, 
that  is  from  b to  f in  Fig.  6.  As  in  the  part  ca, 
the  slope  here  is  always  less  than  that  of  ab,  so  that, 
though  the  densities  may  be  much  greater,  the  con- 
trasts are  less  and  the  negative  is  flat.  The  nega- 


GELATINE  DRY  PLATE 


43 


live  is  so  dense  as  to  require  a long  exposure  for 
printing,  so  that  it  is  hardly  feasible  to  prolong 
development  or  to  intensify  to  increase  contrast. 
In  skillful  hands  reduction  offers  a means  of  im- 
proving the  printing  qualities.  Plates  the  main  part 
of  whose  exposures  fall  on  this  part  of  the  curve, 
that  is  true  overexposure,  occur  less  often  than 
true  underexposure,  since  the  regular  practice  aims 
to  give  an  exposure  lying  close  to  the  lower  limits 
of  correct  exposure. 

30.  Period  of  Reversal. — -This  includes  the  curve 
from  f to  the  right  in  Fig.  6.  Here  increase  of 
exposure  actually  decreases  the  density  obtained  on 
development.  It  is  possible  with  care  to  get  a 
positive,  a flat  poor  one,  but  the  exposures  have  to 
be  very  great  indeed,  so  great  that  one  never  meets 
with  such  reversal  in  ordinary  work.  It  is  of  in- 
terest largely  to  the  investigator  who  is  working  to 
develop  a satisfactory  theory  of  the  latent  image. 

It  is  difficult  to  overestimate  the  importance  of 
the  characteristic  curve  for  a correct  understand- 
ing of  photographic  work. 

It  summarizes  in  a handy,  usable  way  the  most 
important  properties  of  the  dry  plate.  The  previ- 
ous paragraphs  have  indicated  some  of  these  and 
there  are  a number  yet  to  be  discussed. 

31.  Dependence  of  Slope  on  Development. — The 
method  of  obtaining  the  characteristic  curve,  see 


44 


PHOTOGRAPHY 


Art.  25,  was  to  expose  a plate  for  known  different 
times  on  different  spots,  and  then  develop  the  whole 
plate  alike.  Curves  can  be  obtained  in  this  way 
where  the  only  difference  in  the  treatment  of  the 
plate  is  in  different  times  of  development.  Fig. 
7 is  a typical  case  of  two  such  curves  where  the 
developer  used  contained  no  soluble  bromide.  The 


upper  and  steeper  curve  is  for  the  longer  develop- 
ment. The  straight  portions  when  extended  meet 
in  the  point  z which  lies  on  or  near  the  horizontal 
axis.  Hence  as  development  proceeds  the  straight 
part  really  rotates  about  this  point.  If  the  devel- 
opment be  very  prolonged  the  curve  reaches  a maxi- 
mum slope  since  development  finally  ceases,  see  Art. 
19,  and  this  maximum  slope  or  value  of  7 (called 
7oo)  varies  with  different  kinds  of  plates. 

These  observations  are  in  agreement  with  the 


GELATINE  DRY  PLATE 


45 

results  given  in  Art.  21  and  could  have  been  pre- 
dicted from  them.  Equation  5a  was 

d.2  D2 

That  is,  as  development  proceeds  the  different  densi- 
ties always  keep  the  same  proportion.  In  Fig.  7 
from  similar  triangles 

hg  “ yg 

which  is  the  same  statement  as  Equation  5a. 

The  presence  of  soluble  bromide,  potassium  bro- 
mide usually,  in  the  developer  shifts  the  point  z 
below  the  axis.  This  means  suppressing  the  lower 
densities,  and  in  the  case  of  plates  there  appears  no 
compensating  advantage,  so  that  plate  developers 
free  of  bromide  are  to  be  preferred. 

32.  Development  Factor. — -In  Art.  26  it  was 
shown  that 


xy  = 7-yz 


And  therefore  the  value  of  y (gamma)  depends  on 
the  angle  between  these  two  lines,  or  in  other  words 
on  the  slope  of  ab  (Fig.  6).  From  the  preced- 
ing discussion.  Art.  31,  it  will  be  evident  that  the 
value  of  y is  a measure  of  the  amount  of  develop- 
ment, and  it  is  hence  called  the  development  factor. 
For  a true  picture  the  lights  and  shades  must  be 


PHOTOGRAPHY 


46 

proportional  to  those  in  the  subject,  which  requires 
a negative,’’  see  Art.  27,  for  which  y must 

be  unity,  and  the  slope  of  the  line  ab  therefore 
45°.  If  the  development  is  carried  beyond  this,  y 
becomes  greater  than  unity  and  the  contrasts  will 
be  exaggerated.  If  development  is  stopped  short 
of  this,  y will  be  less  than  unity,  and  the  contrasts 
will  be  softened.  In  Art.  86  are  discussed  the  rea- 
sons which  may  lead  to  the  choice  of  a development 
factor  different  from  unity. 

The  amount  of  development,  that  is  the  develop- 
ment factor,  y,  depends  on  a number  of  things,  (a) 
The  developer,  meaning  by  this  the  kind  and  amount 
of  every  constituent.  By  ordinary  care  in  the  mak- 
ing up  of  the  solutions  this  may  be  easily  kept  suffi- 
ciently constant  to  be  relied  upon.  The  choice  of  a 
developer  is  discussed  in  Art.  38.  (b)  The  time  of 

development,  which  is  readily  controlled,  (c)  The 
temperature,  which  may  be  controlled  or  different 
temperatures  may  be  readily  allowed  for  by  chang- 
ing the  time  of  development.  This  will  require 
reading  the  temperature  and  knowing  the  effect  of 
temperature  on  the  rate  of  development.  See  Art. 
33.  (d)  The  dry  plate.  In  order  that  a particular 

brand  of  plates  may  give  uniform  results,  the  time 
of  development  for  y equal  one  must  not  vary  ma- 
terially, and  the  good  plate  makers’  efforts  are  di- 
rected to  maintaining  this  constant  in  different 


GELATINE  DRY  PLATE 


47 


batches.  With  different  brands  of  plates  this  time 
of  development  is  very  different.  The  careful  user 
of  plates  will  welcome  the  time  when  each  box  of 
plates  will  contain  a statement  of  the  plate  speed, 
a formula  for  a good  developer,  and  a table  of  the 
temperatures  and  the  times  of  development  for 
obtaining  y equal  one  with  this  developer,  (e)  The 
age  of  the  plate.  In  moderately  new  plates  which 
have  been  kept  dry  and  cool,  and  away  from  active 
gases,  the  age  is  not  a material  factor.  But  under 
other  circumstances  the  age  of  the  plate  may  intro- 
duce as  much  as  a factor  of  three  in  the  time  of 
development  for  y equal  one.® 

1 33.  Practical  Control  of  the  Amount  of  Develop- 

ment (Contrast). — In  the  dark  room,  two  of  the 
factors  in  the  above  list  offer  difficulty,  that  is  the 
temperature  and  the  plate.  To  manage  all  of  them 
and  so  control  the  contrast,  there  are  several  dis- 
tinct procedures  possible. 

(a)'  Use  always  the  same  brand  of  plate  so  that  it 
may  be  assumed  that  its  development  speed  remains 
constant.  This  will  hold  quite  satisfactorily  for 
plates  of  the  same  batch  of  emulsion  but  the  varia- 
tion of  different  batches  may  have  to  be  compen- 
sated for  by  changing  the  time  of  development;  the 
probabilities  are,  however,  that  in  ordinary  picture 
taking  it  will  require  careful  attention  to  notice  the 

“ Sheppard  & Mees,  “Investigations,”  p.  56. 


PHOTOGRAPHY 


48 

difference.  Develop  all  exposures  for  a fixed  time 
at  a fixed  temperature.  Determine  the  time  of  de- 
velopment by  exposing  several  plates  for  the  same 
time  to  the  same  subject,  and  develop  them  for 
different  times  at  the  fixed  temperature.  From  the 
print  select  the  development  time  for  the  plate  which 
has  the  contrast  you  like.  The  temperature  may  be 
readily  controlled  by  using  a large  tank  of  water 
and  adjusting  its  temperature  as  necessary  by  the 
addition  of  hot  or  cold  water.  The  size  of  the  tank 
will  prevent  its  temperature  changing  quickly.  The 
plates  may  be  developed  in  metal  trays  floated  in 
the  tank,  or  in  a closed  box  immersed  in  the  tank. 
This  general  method  has  been  used  in  considerable 
experimental  work. 

(b)  Again  assume  the  plate  a constant  factor  by 
using  one  brand.  Make  up  the  developer  from 
water  and  concentrated  solutions,  all  of  which  have 
stood  in  the  room  long  enough  to  have  acquired  its 
temperature.  Read  the  temperature  of  the  mixed 
developer  and  by  means  of  a development  time- 
temperature  table  determine  the  time.  In  this 
Art.  a list  of  temperature  coefficients  for  some  of 
the  ordinary  developing  agents  is  given.  The  tem- 
perature coefficient  is  the  factor  by  which  the  speed 
of  development  changes  for  a rise  of  10°  In 

’For  1°  C.  the  coefficient  would  be  the  antdog  of  i/io  the 
log  of  the  coefficient  for  10°  C. 


GELATINE  DRY  PLATE 


49 


another  column  is  given  the  coefficient  for  i°  C. 
If  a suitable  time  giving  the  contrast  desired  has 
been  determined,  as  in  method  (a)  above,  the  time 
for  each  degree  above  (or  below)  the  temperature 
used  in  the  test  may  be  found  by  successive  multi- 
plication (or  division)  by  the  one  degree  coefficient. 
In  this  way  can  be  built  up  a table  giving  the  times 
of  development  for  the  various  temperatures  likely 
to  occur.  This  is  probably  the  most  usable  method 
for  tank  development  and  for  plates  which  are 
sensitive  to  the  dark  room  light  and  must  be  de- 
veloped entirely  in  the  dark.®  It  should  be  observed 
however  that  these  coefficients  are  dependent  some- 
what on  the  plate  used  ^ and  are  not  constant  over  a 
wide  range  of  temperature,  but  when  used  as  above 
the  errors  will  not  ordinarily  be  troublesome. 

(c)  The  Watkins  Factorial  System.^^ — Watkins 
found  by  experiment  that  the  time  taken  for  the 
first  detail  of  the  high  light  of  a negative  to  appear 
when  immersed  in  the  developer,  was  not  dependent 
on  the  exposure  if  it  was  within  the  usual  limits, 
but  dependent  only  on  the  speed  of  development. 

^Wratten  & Mees,  “Development  by  Time,”  British  Journal, 
57>  37^  (1910)  i Watkins’  Photography,  page  105,  D.  Van 
Nostrand  Co.,  N.  Y.  (1911).  This  is  one  of  the  best  texts  now 
published. 

® See  also  appendix,  page  219,  on  development. 

“Watkins’  Manual,  Burke  and  James,  Chicago;  Watkins’ 
Photography,  D.  Van  Nostrand  Co.,  New  York  (1911)  ; 
Mees  and  Wratten,  Brit.  Jour,  of  Photo.,  vol.  54,  p.  560  (1907). 


so 


PHOTOGRAPHY 


In  consequence  this  time  is  a measure  of  the  speed 
of  development,  and  one  only  needs  to  continue  de- 
velopment for  a definite  multiple  of  this  time  to 
reach  the  same  contrast  in  all  cases.  This  multiple, 
called  the  'Watkins  Factor,’"  is  not  dependent  on 
the  plate  used,  nor  on  the  temperature  of  develop- 
ment, nor  within  moderate  limits  on  the  concentra- 
tion of  the  developer,  but  is  dependent  on  the  de- 
veloping agent  used  and  on  the  contrast  desired  in 
the  negative.  See  the  Manual,  Exp.  6,  for  detailed 
directions.  Below  is  a list  of  factors  for  some  of 
the  common  developing  agents  where  the  contrast 
reached  is  0.9  gamma. 


Table  7 


Developing 

agent 

Watkins  Factor 
for  = 0.9 

Temp,  coeff. 
for  10°  C 

Temp,  coeff. 
for  1°  C 

Pyro,  8g/L,  no 

bromide 

9 

1-5 

1. 041 

Metol  Hydro  as 

in  Manual . . . 

10 

1.9 

1.066 

Hydrochinon 

with  Bromide 

5 

2.2 

1.082 

Ferrous  oxalate 

1-7 

1-055 

Rodinal 

40 

1.9 

1.066 

34.  Contrast.  — Contrast  is  defined  as  the  actual 
difference  in  density  between  the  two  points  being 
compared  on  the  negative,  or 

dg  — di  = contrast 


GELATINE  DRY  PLATE 


51 


It  is  not  concerned  with  the  magnitude  of  the  den- 
sity, so  that  a negative  in  general  very  dense  may 
have  very  small  contrast  if  it  has  small  differences 
in  densities  between  the  various  points.  In  a nega- 
tive which  has  been  exposed  so  as  to  fall  within  the 
period  of  correct  exposure,  contrast  depends  on 
three  things: 

(a)  Brightness  differences  in  the  image  al- 
lowed to  fall  on  the  plate.  In  studio  work 
this  is  under  ready  control,  but  in  outdoor  work  the 
only  control  is  by  choice  of  time  of  day  or  of  char- 
acter of  day.  (b)  Development  factor.  As  devel- 
opment proceeds  the  densities  increase  proportion- 
ally so  that  the  differences  increase,  or  contrast 
grows  with  the  progress  of  development.  When 
prolonged,  development  slows  up  or  stops  and  maxi- 
mum contrast  from  this  factor  has  been  obtained, 
(c)  Fog.  It  is  the  result  of  the  reduction  by  the 
developer  of  the  silver  bromide  which  has  not  been 
affected  by  the  light.  Obviously  the  most  fog  will 
be  produced  where  there  is  the  most  unacted  on 
silver  bromide,  that  is,  in  the  thin  parts  of  the  nega- 
tive. Its  effect  will  always  be  to  reduce  the  contrast 
and  it  may  be  a very  material  factor  in  moderately 
long  development. 

Maximum  contrast  is  obtained  by  choosing  a 
plate  having  a large  possible  development  factor 
(process  plates),  exposing  it  to  a brilliantly  lighted 


52 


PHOTOGRAPHY 


subject,  and  developing  it  for  a long  time  in  a de- 
veloper which  produces  as  little  fog  as  possible. 
This  contrast  may  be  increased  greatly  by  intensifi- 
cation. 

35.  Thickness  of  Film.— The  effect  of  the  thick- 
ness of  the  film  on  the  glass  plate  is  shown  by  the 
curves  of  Fig.  8.  For  a given  exposure  and 


amount  of  development,  the  density  depends  on  the 
thickness  of  the  film;  in  general  the  thicker  the  film 
the  greater  the  density.  The  thinner  coated  plates 
have  a much  shorter  straight  part  to  the  curve,  that 
is,  the  period  of  correct  exposure  (called  latitude) 
is  much  shorter,  the  exposures  have  to  be  more  ac- 
curately timed  to  fall  within  it,  and  it  may  be  even 


GELATINE  DRY  PLATE 


S3 


incapable  of  rendering  strong  contrasts.  The  thin- 
ner coated  plate  is  however  a trifle  faster,  but  the 
greater  speed  is  obtained  at  too  great  a cost,  much 
the  better  all  round  plate  is  the  one  with  a generous 
amount  of  the  silver  salt  per  unit  of  area. 

36.  Like  Negatives  from  Different  Exposures 
(Latitude). — Suppose  a number  of  negatives  be  made 
giving  each  a different  exposure  but  yet  such  a one 
that  it  falls  completely  within  the  period  of  correct 
exposure.  All  are  developed  for  the  same  length  of 
time  in  the  same  developer.  This  will  make  all  the 
plates  fall  on  the  same  characteristic  curve.  The 
longer  exposure  given  a plate  merely  multiplies  the 
exposure  everywhere  on  the  plate  without  affecting 
the  proportion  between  the  various  spots  on  the 
plate.  That  is,  the  amount  of  light  coming  from 
the  brightest  object  will  be  the  same  multiple  of  that 
from  the  weakest  object  for  each  of  the  different 
plates.  In  the  regular  characteristic  curve  diagram 
equal  multiples  of  exposure  are  plotted  as  equal 
horizontal  shifts.  Hence  in  the  diagram  Fig.  9, 
if  xy  are  the  limits  of  exposure  for  one  plate, 
x'y'  will  be  for  the  other,  and  the  distances  xy  and 
x'y'  will  be  alike.  Then  the  distances  be  and 
b'c'  will  also  be  equal.  That  is,  the  differences  in 
the  densities  (contrast)  in  the  two  plates  are  alike 
and  they  will  give  identical  prints.  One  plate  will 
be  denser  than  the  other,  a'x'  being  greater  than  ax, 


54 


PHOTOGRAPHY 


and  will  require  a longer  printing  exposure,  but  if 
the  printing  exposures  are  made  such  that  the  same 
amount  of  light  gets  through  to  the  paper  in  the 
two  cases  and  the  papers  are  developed  alike,  the 
resulting  prints  will  be  identical/^ 


This  result  is  very  important,  as  it  means  that  it 
makes  no  difference  in  the  print  where  the  plate 
exposure  lies  within  the  period  of  correct  exposure. 
The  latitude  of  good  plates  is  large,  about  a factor 
of  6o.  Outdoors,  in  sunlight,  the  sky  will  be  about 
30  times  as  bright  as  a piece  of  black  velvet.^^  An 
interior  picture  including  a window  will  exceed  this 
considerably.  On  the  other  hand,  subjects  which 
do  not  include  any  brightly  lighted  white  articles 

“ See  series  of  prints  published  by  F.  D.  Todd  in  Photo  Be(p- 
con  16,300,  Nov.,  1904,  where  the  plate  exposure  was  Varied 
from  I to  16  and  the  prints  are  indistinguishable. 

“ C.  E.  K.  Mees,  Wilson* s Photographic  Magazine,  Dec.,  1913. 


GELATINE  DRY  PLATE 


55 


or  very  dark  ones  will  be  considerably  less  than  this 
which  gives  considerable  margin,  a factor  of  at  least 
4,  often  more  than  lo,  in  estimating  exposures 
within  which  the  prints  will  be  the  same. 

37.  Organic  dry  plate  developers  are  very  much 
alike.  The  active  material  is  some  good  reducing 
substance  like  pyrogallic  acid,  metol,  hydrochinon, 
glycin,  amidol,  and  so  on  for  several  hundred.  Too 
strong  a reducing  substance  will  attack  the  whole 
of  the  silver  salt,  so  that  the  substance  used  has  to 
be  of  a reducing  power  such  as  to  distinguish  be- 
tween the  two  states  of  the  silver  salt.  Experiment 
has  shown  that  there  is  nothing  gained  in  having 
it  present  in  greater  proportion  than  a few  grams 
per  liter,  for  example  about  4g  per  L is  the  maxi- 
mum useful  amount  of  pyro.  A few  of  the  devel- 
oping agents  work  without  the  simultaneous  pres- 
ence of  an  alkali  (amidol,  dianol).  Most  organic 
developing  agents  require  the  presence  of  some  al- 
kaline substance  to  cause  it  to  become  active.  It  is 
possible  that  the  hydrobromic  acid  liberated  by  the 
reaction  prevents  further  reduction  unless  it  is  neu- 
tralized. Almost  any  alkaline  material  will  serve, 
the  usual  ones  being  the  carbonates  or  hydrates  of 
potassium  or  sodium.  The  carbonates  are  to  be 
preferred,  as  they  are  not  as  destructive  of  every- 
thing they  get  on  including  the  skin.  The  speed 
with  which  the  developer  acts  depends  more  on  the 


PHOTOGRAPHY 


56 

concentration  of  the  alkaline  substance  than  on  that 
of  the  reducing  agent  itself. 

As  a rule  most  developing  agents  stain  a little, 
some  stain  vigorously,  and  the  sulphite  has  the  prop- 
erty of  holding  up  this  staining  habit,  but  most  de- 
velopers will  work  well  without  it.  Unfortunately 
the  sulphite  adds  strongly  to  the  fogging  properties, 
but  this  fogging  may  be  prevented  for  the  ordinary 
times  of  development  by  the  presence  of  a definite 
amount  of  potassium  bromide.  This  latter  is  called 
the  restrainer,  as  it  also  slows  up  the  action  of  the 
developer  on  the  latent  image.  In  a general  way  the 
velocity  of  development  depends  on  the  concentra- 
tion of  the  developer,  and  halving  the  concentra- 
tion, that  is,  doubling  the  volume  by  adding  water, 
will  double  the  time  necessary  for  the  same  contrast, 
that  is  for  the  same  development  factor.  In  the  use 
of  the  tank  one  can  calculate  the  time  required  as 
inversely  as  the  dilution,  though  it  is  only  approxi- 
mately true  and  will  not  hold  over  wide  ranges. 

38.  The  choice  of  a developer  will  turn  upon 
several  factors.  It  should  be  cheap,  easily  com- 
pounded, keep  well  when  made  up,  produce  very 
little  fog,  and  not  stain  the  gelatine,  paper,  or  fin- 
gers. There  should  be  sufficient  of  the  active  agent 
present  to  complete  the  development  of  a dense 
plate  without  the  solution  becoming  exhausted.  The 
alkali  should  be  adjusted  so  that  the  time  required 


GELATINE  DRY  PLATE 


57 


is  a convenient  one,  three  to  seven  minutes, — so 
that  the  time  required  for  immersing  the  plate  in 
the  developer,  or  for  changing  it  to  the  hypo  will 
not  be  a material  fraction  of  the  time  of  develop- 
ment If  sulphite  is  present  it  should  be  there  in 
sufficient  quantity  to  hold  up  the  staining  for  the 
time  required  for  the  development.  In  paper  de- 
velopers the  presence  of  the  sulphite  will  require  the 
presence  of  bromide  to  hold  up  the  sulphite  fog 
which  would  gray  the  whites.^^  The  effect  of  the 
bromide  on  the  lower  gradations  makes  it  advisable 
to  omit  it  from  the  plate  developers.  Developers 
for  use  in  tanks  must  be  chosen  with  special  refer- 
ence to  their  resistance  to  air  oxidation  and  to  free- 
dom from  staining  habits.  In  all  cases  the  smaller 
the  fogging  property  the  better.  The  developer 
advised  by  the  maker  of  the  plates  or  paper,  while 
not  necessarily  the  best  in  each  case,  may  be  de- 
pended on  to  give  good  results. 

39.  Estimation  of  Exposure. — Exposure  is  de- 
fined as  the  ratio  between  the  amount  of  light  and 
the  area  on  which  it  falls,  or  in  other  words  the 
amount  of  light  per  square  centimeter.  It  cannot 
be  too  strongly  emphasized  that  to  make  a satisfac- 
tory negative  the  plate  must  be  correctly  exposed, 
that  is  all  the  different  amounts  of  light  from  the 

“Mees  & Piper,  “Fogging  Power  of  Developers,”  British 
Journal,  58,  491  (1911). 


PHOTOGRAPHY 


58 

different  parts  of  the  subject  must  expose  the  cor- 
responding part  of  the  plate  so  as  to  fall  within 
the  latitude.  The  latitude  includes  the  exposures 
falling  on  the  straight  part  of  the  characteristic 
curve,  and  a limited  amount  of  the  curved  parts 
may  be  included  without  noticeable  loss.  This  leaves 
a good  margin  with  almost  all  subjects  and  most 
plates.  See  Art.  36.  The  different  factors  affect- 
ing the  exposures  are:  (a)  the  time  for  which  the 
camera  remains  open,  (b)  the  stop  used,  (c)  the 
speed  of  the  plate,  (d)  the  character  of  the  sub- 
ject, and  (e)  the  lighting.  The  first  three,  time, 
stop,  and  plate  speed,  practically  are  easily  man- 
aged and  fairly  constant  factors,  requiring  only  a 
little  experience  to  make  their  management  certain. 
The  subject  is  often  deceptive  and  uncertain,  but 
judgment  of  the  lighting  is  of  all  the  most  continu- 
ous and  persistent  difficulty.  All  these  factors  are 
discussed  in  detail  below. 

40.  Shutters. — The  time  for  which  the  camera 
remains  open  depends  usually  on  the  mechanism 
called  the  shutter.  When  used  merely  as  a means 
of  opening  and  closing  the  lens  (''time’’  and  "bulb” 
settings)  they  are  usually  reliable,  though  they  will 
occasionally  fail  to  open  or  close  as  expected,  so 
that  some  experienced  workers  refuse  to  use  any 
save  the  very  simplest,  most  reliable  types. 

There  are  a number  of  different  types  of  shut- 


GELATINE  DRY  PLATE 


59 


ters,  drop-curtain,  revolving  sector,  roller-blind, 
diaphragm,  all  of  which  are  placed  close  to  the 
lens  either  in  front,  behind  or  between  the  compo- 
nents. Undoubtedly  the  most  consistent,  accurate, 
and  efficient  are  the  roller-blind,  focal-plane  shut- 
ters. They  can  be  fitted  to  almost  any  camera,  but 
are  expensive,  somewhat  bulky,  and  less  simple  to 
use.  In  these  a roller  blind,  with  a slit  of  variable 
width  across  it,  passes  just  in  front  of  the  plate  at  an 
adjustable  speed.  They  are  the  only  fully  depend- 
able shutters  for  speeds  above  1/50  second. 

The  great  majority  of  cameras  are  fitted  with  be- 
tween-the-lens  shutters,  in  which  the  iris  diaphragm 
opens  and  shuts,  or  separate  blades  move  to  open 
and  close  the  lens. 

There  are  several  ways  in  which  shutters  give 
trouble.  The  most  difficult  to  deal  with  is  lack  of 
consistency.  They  do  not  always  give  the  same 
duration  of  opening  when  set  in  the  same  way. 
Most  shutters  now  use  the  escape  of  air  from  a 
partly  closed  cylinder  as  the  means  to  control  the 
time.  To  obtain  rapid  motion  without  jarring  the 
camera,  the  moving  parts  have  to  be  very  light  and 
the  springs  weak,  under  which  circumstances  fric- 
tion becomes  a very  difficult  thing  to  keep  constant. 

The  time  markings  are  usually  intended  to  mean 
the  time  between  the  first  and  last  appearance  of 
opening.  With  a moving  subject  the  duration  of 


6o 


PHOTOGRAPHY 


exposure  must  be  short  enough  that  the  blurring 
produced  in  the  picture  will  be  barely  visible.  Very 
few  shutters  are  reasonably  true  to  the  times 
marked.  The  highest  speeds  seldom  exceed  1/35 
second,  that  is  the  settings  marked  1/25,  1/50, 
1/ 1 00,  all  give  1/20  to  1/35  second.  The  slower 
speeds  can  be  adjusted  to  offset  the  aging  of  the 
spring  and  the  wear  on  the  bearings  but  without 
any  great  effect  on  the  higher  speeds. 

With  this  way  of  marking  the  times,  no  account 
is  taken  of  the  fact  that  the  shutter  requires  time 
to  open  and  time  to  close.  For  speeds  of  one  sec- 
ond the  proportion  of  the  time  spent  in  opening  and 
closing  is  small,  so  that  the  amount  of  light  getting 
to  the  plate  is  almost  as  great  as  if  the  time  spent 
in  opening  and  closing  was  zero,  and  the  efficiency 
is  therefore  nearly  unity.  But  as  one  goes  to  the 
higher  speeds  the  proportion  of  the  time  spent  in 
opening  and  closing  becomes  greater,  so  that  the 
efficiency  may  drop  as  low  as  25%.  In  many  shut- 
ters the  higher  speeds  are  limited  by  the  time  re- 
quired for  opening  and  closing,  and  the  lens  is  at 
full  aperture  only  a very  small  fraction  of  the  total 
time.  One  can  conclude  that  the  reliability,  the  accu- 
racy and  the  efficiency  are  none  too  good  and  that 
they  are  often  unsuspected  but  prolific  sources  of 
trouble. 

The  complete  testing  of  a shutter  for  ac- 


GELATINE  DRY  PLATE 


6i 


curacy  and  efficiency  is  a complicated  matter.  Suf- 
ficient information  for  ordinary  use  may  be 
obtained  by  exposing  a plate  in  sections  by  each 
time  covering  the  rest  of  the  plate  with  black  paper. 
Use  a uniform,  evenly  lighted  subject — a white 
brick  wall  for  instance — and  change  the  shutter 
setting  for  each  section,  at  the  same  time  changing 
the  stop  so  as  to  offset  the  change  in  time.  On 
development  all  the  sections  should  be  uniform  in 
density.  If  they  are  not  the  trouble  is  almost  cer- 
tainly with  the  shutter,  as  the  diaphragm  can  usu- 
ally be  depended  on.  By  measuring  the  density  in 
the  photometer  one  can  determine  how  much  the 
shutter  is  wrong  in  the  matter  at  least  which  con- 
cerns the  user  most,  namely  the  total  amount  of 
light  admitted. 


41.  The  Stop. — The  system  of  numbering  ap- 
plied to  stops  requires  a very  detailed  explana- 
tion. 

In  the  figure  let  ab  represent  the  diaphragm  in 
the  lens  through  which  the  light  passes  to  the  plate 


62 


PHOTOGRAPHY 


CE.  Consider  a single  point  in  the  opening  ab; 
from  it  the  light  will  spread  out  in  a cone  to  cover 
CE.  The  amount  of  light  falling  on  a square  centi- 
meter of  CE  will  evidently  depend  on  the  distance  of 
CE  from  ab;  the  greater  the  distance  the  greater 
the  area  over  which  the  light  must  be  spread.  The 
area  increases  as  the  square  of  the  distance  x in- 
creases, so  that 

B : i/x2 

where  B is  the  brightness  of  the  image.  This  will 
evidently  be  true  for  every  point  and  consequently 
for  the  whole  area  ab. 

The  amount  of  light  entering  ab  will  depend  on 
the  area  of  the  opening  (usually  circular)  increas- 
ing as  this  area  increases,  that  is 

B :d2 

where  d is  the  diameter  of  the  circular  stop  ab. 
These  two  proportions  may  be  combined,  thus 

B : dVx^ 

For  the  ordinary  use  of  the  lens  in  taking  pictures 
(not  in  enlarging)  x is  so  close  to  the  focal  length 
of  the  lens  that  for  this  work  we  may  replace  x by 
F,  and  write 

B : dVF^ 

Hence  if  we  make  the  diaphragm  diameter  the  same 
proportion  of  the  focal  length  of  the  lens  it  is  to 


GELATINE  DRY  PLATE  63 

be  used  with,  we  may  shift  from  one  lens  to  another 
without  changing  the  brightness  of  the  image  and 
consequently  without  changing  the  time  required 
for  exposure.  For  this  reason  the  diaphragm  is 
always  referred  to  as  a proportion  of  the  focal 
length.  These  proportions  are  what  are  called  the 
''F  numbers,’’  thus  ''F16”  or  more  strictly  ''F/16,” 
means  that  the  diameter  of  the  diaphragm  referred 
to  is  1/16  of  the  focal  length.  A factor  of  2 in 
the  F number  will  mean  a factor  of  4 in  the  image 
brightness ; see  the  above  proportion.  For  ordinary 
use  a factor  of  2 is  very  convenient,  so  that  a 
series  of  F numbers  are  arranged  such  that  the 
image  brightness  changes  by  a factor  of  2 and  there- 
fore the  F number  by  a factor  of  the  square  root 
of  2.  These  F numbers  are  given  in  the  following 
table  in  the  left  hand  column. 


Table  8 


F numbers 

U.  S.  nm 

4 

I 

5-65 

2 

8 

4 

8 

i6 

16 

22.6 

32 

32 

64 

45-2 

128 

64 

256 

The  series  in  the  right  hand  column  is  called  the 
Universal  System  numbers  and  is  a little  simpler 


64 


PHOTOGRAPHY 


in  use.  The  number  i6  is  chosen  to  agree  with  the 
F number,  and  from  this  point  the  numbers  run 
both  up  and  down,  always  changing  by  factors  of 
2 in  the  number  and  in  the  image  brightness,  but 
an  increase  in  the  number  corresponds  to  a decrease 
in  the  brightness,  and  therefore  also  to  an  increase 
in  the  time  of  exposure  to  keep  the  exposure  the 
same.  Shutters  may  be  obtained  marked  with 
either  series  of  numbers,  the  U.  S.  probably  being 
the  more  usual  in  this  country.  When  referring 
I to  the  speed  of  a lens  the  F number  corresponding 
I to  the  largest  stop  which  can  be  used  with  the  lens 
^ is  usually  given. 

42.  Dry  Plate  Speeds.^^ — By  the  speed  of  a dry 
plate  is  meant  the  ratio  between  the  opacity  in  the 
negative  and  the  exposure  required  to  produce  that 
opacity.  A fast  plate  is  one  which  for  a small  ex- 
posure will  give  a dense  negative,  that  is  the  ratio 
0/E  is  large.  Plate  speeds  vary  a great  deal.  Not 
only  do  they  vary  when  they  are  meant  to,  as  in 
different  makers’  plates  or  in  different  brands  from 
the  same  maker,  but  different  plates  of  the  same 
brand  or  even  from  the  same  batch  will  vary  some- 
what. For  speed  determination  it  is  necessary  to 
find  a reproducible  light  which  shall  bear  some  defi- 
nite relation  to  daylight,  to  fix  the  conditions  of 

Watkins’  Photography,  page  297;  Sheppard  & Mees,  Inves- 
tigations,  p.  278. 


GELATINE  DRY  PLATE  65 

exposure  and  of  development,  and  to  test  the  result- 
ing negative. 

(a)  Hurter  and  Driffield’s  work  has  led  to  the 
most  satisfactory  method  of  measuring  and  ex- 
pressing the  speed  of  plates,  but  it  still  leaves  the 
difficulty  over  the  light.  In  Art.  27  Equation  6 
gives  the  relation  between  opacity  and  exposure  as 

E/i  = O 


where  i is  the  distance  of  the  point  z from  the  origin 
and  is  best  expressed  in  exposure  units.  This  equa- 
tion is  true  only  for  a development  factor  of  unity. 
Also 


1/i  = 0/E 


from  which  it  appears  that  1/i  is  a measure  of  the 
speed  of  the  plate.  A somewhat  different  argu- 
ment will  make  this  even  more  evident.  In  Art. 
31  is  described  the  way  the  characteristic  curve 
shifts  with  time  of  development,  which  for  devel- 
opers not  having  any  potassium  bromide,  makes 
the  common  point  z lie  on  the  axis.  Fig.  ii  gives 
another  example  taken  from  the  later  work  of 
Sheppard  and  Mees.  The  distance  of  z from  the 
origin  is  the  quantity  above  called  i;  i is  therefore 
the  exposure  which  is  necessary  to  bring  the  densi- 
ties onto  the  straight  part  of  the  curve  and  is  called 
the  ‘‘inertia”  of  the  plate.  Obviously  if  the  inertia 
is  large  the  plate  will  be  a slow  one  as  it  will  take 


66 


PHOTOGRAPHY 


a large  exposure  to  bring  the  plate  to  the  useful 
part  of  the  curve.  Conversely  for  small  inertias. 
From  all  which  it  appears  that  the  inertia,  i,  is  ac- 
curately inversely  proportional  to  the  speed  of  the 
plate.  The  ‘'H  and  D number’’  often  given  with 


English-made  plates  is  the  constant  34  divided  by 
the  inertia.^®  American  plate  manufacturers  and 
photographers  have  hardly  used  this  system  at  all. 

(b)  In  this  country  the  usual  number  one  sees 
referring  to  the  speed  of  the  plates  is  the  Watkins 
number  for  use  with  his  meter.  This  is  based  on 
calling  a plate  unit  speed  when  it  takes  two  seconds 
in  bright  sunlight,  in  midday  in  June,  with  stop  8 to 
make  a minimum  correct  exposure.  This  is  a very 
"See  appendix  on  Plate  Speed  Systems. 


GELATINE  DRY  PLATE  67 

slow  plate,  and  the  numbers  run  from  about  20  for 
a slow  process  plate  to  about  500  for  the  fastest. 
The  numbers  increase  with  the  speed  and  hence  the 
time  required  for  an  exposure  is  in  the  inverse  ratio, 
the  exposure  decreasing  as  the  number  increases. 
These  numbers  are  mostly  useful  for  comparison, 
and  one  has  to  remember  some  exposure  to  start 
with  as  correct,  and  then  modify  it  according  to  the 
plate,  time  of  day,  etc.  Besides  these  systems  of 
referring  to  the  speed  of  plates  there  are  two  others 
one  sees  in  English  works,  called  the  Warnercke 
number  and  the  Wynne  number. 

43‘  In  connection  with  the  speed  of  plates,  there 
is  an  interesting  deduction  from  the  characteristic 
curve  as  to  a method  of  increasing  the  speed  some- 
what. Every  part  of  the  plate  should  receive 
enough  exposure  to  carry  it  to  the  straight  part  of 
the  curve,  that  is,  an  exposure  corresponding  to  the 
inertia.  After  that  the  light  from  the  subject  be- 
comes really  effective  in  building  up  the  desired 
latent  image.  Evidently  one  could  give  a uniform 
exposure  all  over  the  plate,  for  example  by  opening 
the  holder  to  weak,  artificial  light  for  a short  time; 
then  the  picture  one  wants  can  be  taken  with  less 
exposure  to  the  subject— that  is  the  speed  of  the 
plate  is  increased.  The  increase  however  is  small 
and  hardly  worth  the  trouble,  except  in  unusual 
cases,  as  in  astronomy,  where  the  observer  has  to 


68 


PHOTOGRAPHY 


hold  the  object  to  be  photographed  on  the  cross 
hairs  of  the  telescope  so  that  the  image  will  stay 
at  the  same  place  on  the  plate,  often  for  hours  or 
all  night,  so  that  an  increase  of  a few  per  cent  in 
the  speed  of  the  plate  will  be  welcome. 

The  speed  of  dry  plates  are  also  connected  some- 
what  with  what  is  called  the  ''grain.’’  It  will  be 
recalled  from  the  way  in  which  the  emulsion  was 
made  that  the  silver  bromide  will  be  in  the  form  of 
little  discreet  particles,  and  when  the  bromide  is 
reduced  to  silver  in  development,  the  silver  parti- 
cles will  correspond  somewhat  with  the  bromide 
particles.  These  silver  particles  are  what  is  called 
the  grain  of  the  plate.  The  size  varies  greatly,  de- 
pending on  the  method  of  making  the  emulsion,  the 
time  of  boiling,  etc.,  the  fast  plates  having  almost 
invariably  the  larger  grain.  In  the  slower  plates 
a microscope  is  required  to  make  them  visible  but  in 
some  of  the  fast  plates  they  are  almost  visible  to 
the  unaided  eye,  and  under  these  circumstances  will 
affect  the  geometry  of  the  picture  somewhat.  A 
fine-grained  plate  is  much  to  be  preferred.^® 

44.  Character  of  Subject. — The  subject  may  af- 
fect the  exposure  in  several  ways,  by  its  color,  in- 
cluding white,  gray  and  black,  by  its  shadows,  and 

“Microscopic  studies  of  the  dry  plate  are  of  great  interest. 
See  series  of  articles  by  Dr.  W.  Scheffer,  British  Journal,  54, 
540  (1907)  and  55,  472  (190B). 


GELATINE  DRY  PLATE  69 

by  its  distance.  As  will  be  discussed  in  detail  later, 
Art.  47,  the  ordinary  silver  bromide  emulsion  is 
most  sensitive  to  the  blue  light  and  least  sensitive 
to  the  deep  red.  With  the  ordinary  plate,  photo- 
graphs of  red  objects,  a red  brick  building  for  ex- 
ample, have  to  be  taken  by  the  scattered  white  light 
which  is  always  mixed  with  the  colored  light  from 
any  object.  In  consequence  red  objects  need  a 
markedly  long  exposure.  This  is  particularly  true 
when  there  is  detail  in  the  red  which  it  is  desired 
to  have  rendered.  The  best  remedy  is  discussed 
later  under  color  sensitiveness.  Art.  51.  Gray  or 
black  objects,  or  any  dark-colored  objects  except 
blue,  shadows  where  the  illumination  is  weak  as  in 
deep  windows  or  doors,  under  trees  or  under  the 
brim  of  a hat,  all  send  a relatively  small  amount  of 
light  to  the  camera,  and  the  general  exposure  of  the 
whole  subject  has  to  be  increased  so  that  these  places 
will  get  sufficient  exposure  to  record  their  detail. 
If  the  picture  includes  any  near  foreground  it  is 
apt  to  be  less  exposed  than  the  rest  of  the  subject, 
and  any  very  distant  detail  is  apt  to  be  overlaid 
with  a fog  due  to  the  blue  haze  always  present  more 
or  less  in  the  atmosphere  probably  due  to  fine  dust. 

45.  Lighting. — For  some  typical  subjects  with 
the  ordinary  fast  plate  (200  Watkins)  the  actual 
exposure  in  June,  between  9 a.m.  and  3 p.m.,  with 


70  PHOTOGRAPHY 

the  sun  shining,  and  using  stop  i6,  are  about  as  fol- 
lows: 

Average  subject  outdoors,  street  scenes,  figures. . 1/15  sec. 

Light  open  landscape  perhaps  including  some 

water 1/30 

Sea,  clouds  and  sky,  snow 1/150 

Dark  outdoor  subjects,  such  as  trees H to  i 

Indoors  in  well  lighted  room i to  10  “ 

Such  a table  can  only  be  a general  guide.  In  cloudy 
weather  these  times  must  be  multiplied  by  a factor 
up  to  4 or  6,  depending  on  the  amount  of  cloudiness. 
The  following  table  will  serve  as  a guide  to  the 
effect  of  the  time  of  day  and  the  time  of  year. 


Table  9 


a.m.  p.m. 

June 

May 

April 

March 

Feb. 

Jan. 

Dec. 

July 

Aug. 

Sept. 

Oct. 

Nov. 

1 2 or  12 

I 

I 

1.25 

1-5 

2 

3.5 

4 

II  I 

I 

I 

1.25 

1. 5 

2.5 

4 

5 

10  2 

I 

I 

1.25 

1.75 

3 

5 

6 

9 3 

I 

1-25 

1-5 

2 

4 

12 

8 4 

1-5 

1.5 

2 

3 

10 

7 “ 5 

2 

2.5 

3 

6 

6 “ 6 

2.5 

3 

6 

5 “ 7 

5 

6 

To  keep  such  data  as  this  in  portable  form  and 
to  help  in  the  calculations  there  are  a great  number 
of  exposure  calculators  and  exposure  meters  on  the 
market.  They  are  mostly  applications  of  the  slide- 
rule  principle,  one  scale  taking  account  of  the  dif- 
ference in  subjects,  others  of  the  time  of  day,  of 


GELATINE  DRY  PLATE 


71 


the  time  of  year,  of  the  speed  of  plates,  and  of  the 
stop.  Another  class  of  these  exposure  meters  in- 
cludes a device  for  measuring  the  photographic 
value  of  the  light.  This  is  accomplished  in  two  dif- 
ferent ways.  In  one  class,  which  includes  the  Wat- 
kins, the  Wynne,  and  the  Johnson  meters,  the  light  is 
allowed  to  fall  upon  a piece  of  light  sensitive  paper 
and  the  time  noted  which  it  requires  to  darken  to 
match  a neighboring  gray  patch.  This  time  is  a 
measure  of  the  photographic  value  of  the  light  and 
is  used  to  calculate  the  exposure.  In  the  other  class, 
which  includes  Heydes  Actino- Photometer,  a wedge 
of  blue-gray  glass  is  placed  in  front  of  one's  eye, 
and  by  shifting  to  the  darker  end  of  the  wedge  the 
detail  observed  begins  to  become  invisible.  The 
position  of  the  wedge  at  which  this  occurs  gives  a 
measure  of  the  amount  of  light  and  is  readily  used 
to  calculate  the  exposure.  The  second  class  has 
some  points  of  superiority — greater  rapidity  in 
weak  light,  measurement  of  the  light  coming  from 
the  subject,  not  of  that  falling  on  it,  freedom  from 
variation  in  the  sensitive  paper,  but  on  the  other 
hand  it  depends  on  the  constancy  of  the  eye.  Any 
of  these  light  meters  are  very  satisfactory  helps  and 
will  quickly  pay  for  themselves  in  plates  saved,  but 
they  have  to  be  used  with  judgment,  as  they  cannot 
take  care  of  the  great  variety  of  actual  circum- 
stances. 


CHAPTER  III 


PROPERTIES  OF  THE  GELATINE  DRY  PLATE^ 
COLOR  SENSITIVENESS 

46.  Anyone  using  the  ordinary  silver  bromide 
plates  will  soon  notice  certain  peculiarities  when 
they  are  used  for  making  the  usual  monachrome 
(black  and  white)  prints  of  colored  objects.  Blue 
objects,  even  dark  blue  ones,  will  appear  almost 
white  in  the  print,  while  red,  yellow,  and  green  will 
appear  in  this  order,  from  black  to  gray.  The 
brightness  of  the  image  for  different  colors  is  not 
at  all  as  the  eye  sees  them.  There  is  also  a second 
difficulty  in  many  colored  subjects.  If  the  expo- 
sures are  made  right  to  show  the  different  shades 
of  blue,  then  the  reds  are  uniformly  black,  showing 
no  difference  between  the  different  shades  of  red, 
that  is  giving  no  detail  in  the  red.  By  making  very 
much  longer  exposures  a certain  amount  of  detail 
can  be  obtained  in  the  red,  but  then  all  the  detail  in 
the  blue  objects  will  be  lost;  they  will  appear  uni- 
formly white.  The  order  of  decreasing  action  of 
the  different  colors  in  making  their  record  on  the 
plate  is,  violet,  blue,  green,  yellow,  orange,  red, 

72 


COLOR  SENSITIVENESS 


73 


that  is  practically  the  short  to  the  long  wave  lengths 
in  the  visible  spectrum.  This  is  not  the  order  at  all 
for  visual  color  brightness,  yellow  being  the  bright- 
est color.  As  a matter  of  .fact,  the  ordinary  plate 
does  not  render  visual  color  brightness  at  all.  We 
have  become  so  used  to  black  and  white  pictures 
showing  the  shadows  and  the  geometric  form,  that 
the  lack  of  this  less  important  detail  is  not  felt, 
unless  the  object  be  very  brightly  colored. 

It  will  hardly  be  necessary  to  point  out  that 
color  is  an  attribute  of  light,  that  the  color  of  an 
object  is  merely  the  result  of  its  power  of  reflect- 
ing some  of  the  colors  of  light  and  absorbing  others. 
Most  colored  objects  reflect  light  which  is  a mixture 
of  colored  lights,  and  we  call  it  an  impure  color  as 
distinct  from  a pure  color  due  to  one  wave  length 
of  light.  To  investigate  this  color  rendering  of 
the  plate  it  is  advisable  to  use  pure  colors  and  get 
their  effect  on  the  plate,  and  then  the  effect  of  the 
impure  colors  will  be  the  sum  of  the  effects  of  the 
different  colors  composing  them. 

47.  Color  Sensitiveness  Curves. — There  are  va- 
rious means  of  separating  the  different  pure  colors 
composing  white  light  into  a band  of  pure  colors 
called  the  spectrum.  The  best  of  these  means  for 
our  purpose  is  probably  the  one  we  use  in  the  lab- 
oratory, a grating,  since  it  spreads  the  colors  out 
evenly  all  along  the  spectrum.  Reference  must  be 


74 


PHOTOGRAPHY 


made  to  any  text  book  on  physics  for  an  explana- 
tion of  its  action.  Accepting  this  grating  spectrum 
then  as  giving  us  the  sum  total  of  the  different 
colors  of  white  light  distributed  in  strict  proportion 
to  the  wave  length,  it  will  only  be  necessary  to  let 
this  band  of  color  fall  upon  the  sensitive  plate  and 
then  develop  it  as  usual,  in  order  to  test  the  effects 
of  the  different  colors.  To  get  the  greatest  amount 
of  information  it  is  best  to  make  a series  of  increas- 
ing exposures  so  that  the  part  of  the  light  which  has 
only  a small  action  will  be  able  to  record  that  which 
it  has.  For  the  ordinary  silver  bromide  plate  the 
action  is  greatest  in  the  blue-violet,  decreasing  rap- 
idly to  almost  nothing  in  the  yellow  and  red  and 
only  a little  less  rapidly  in  the  violet  and  the  ultra 
violet.  If  we  plot  density  vertically,  and  color  (bet- 
ter wave  length)  horizontally,  we  will  have  a curve 
which  will  represent  the  color  sensitiveness  of  the 
plate,  since  the  sensitiveness  is  approximately  pro- 
portional to  the  density  produced.  In  the  same  way 
we  could  measure  and  represent  the  color  sensitive- 
ness of  plates  made  of  the  other  silver  haloids,  sil- 
ver chloride  and  silver  iodide;  see  Fig.  12.  In  the 
visible  part  of  the  spectrum  they  do  not  differ  very 
greatly  from  each  other. 

48.  Dyes.— Some  time  before  gelatine  plates 
were  known,  a German  worker.  Professor  Vogel, 
discovered  that  treating  the  collodion  plates  before 


COLOR  SENSITIVENESS 


75 


exposing  with  certain  dyes  changed  their  color  sensi- 
tiveness very  markedly.  Since  his  work  the  number 
of  dyes  known  to  have  such  action  has  increased 
very  greatly.^  It  was  a very  direct  experiment  to 


Fig.  12.  Curve  A is  the  color  sensitiveness  of  silver  bromide 
gelatine  emulsion.  Curve  B that  of  silver  chloride  gelatine 
emulsion.  Curve  C that  of  silver  iodide  gelatine  emulsion. 

try  the  same  thing  with  gelatine  dry  plates,  where 
they  were  found  to  have  the  same  kind  of  action. 
It  has  been  foimd  that  the  dye  has  to  actually  color 
the  silver  bromide  particles  themselves ; merely  col- 

*R.  J.  Wall,  “Recent  Work  in  Color  Sensitizing,’^  British 
Jour.,  54,  365  (1907). 


PHOTOGRAPHY 


76 

oring  the  gelatine  does  not  give  the  action  referred 
to.  That  is  only  those  dyes  which  are  actually  ab- 
sorbed by  the  silver  haloid  change  the  color  sensi- 
tiveness. The  amount  of  the  dye  required  is 
very  small;  the  usual  bathing  solution  for  all  the 
dyes  is  about  i part  in  100,000  water.  As  a matter 
of  fact,  the  amount  of  staining  of  the  silver  bro- 
mide depends  very  little  on  the  amount  of  dye  in 
the  solution ; the  particles  absorb  so  much  and  then 
will  absorb  no  more.  From  this,  as  one  would  ex- 
pect, it  is  very  difficult  to  wash  the  dye  out,  prac- 
tically impossible  in  fact.  Enough  of  it  to  give  a 
very  noticeable  effect  on  the  color  sensitiveness  re- 
mains there  after  prolonged  washing.  The  dye  so- 
lution is  composed  of  very  fine  particles  of  the  solid 
dye  suspended  in  water  (colloidal  solution)  and 
the  smaller  these  particles  are,  the  better  they  are 
absorbed  by  the  bromide.  Dilution  of  these  solu- 
tions divides  the  particles,  which  is  one  reason  that 
a dilute  solution  works  better  than  a more  concen- 
trated one. 

Only  a few  of  the  very  great  number  of  known 
dyes  have  this  property,  and  the  dyes  have  been  very 
extensively  tested,  two  workers  recording  the  test- 
ing of  600  dyes,  for  example.  There  is  not  any 
apparent  rule  as  to  what  dyes  will  prove  useful  for 
this  purpose,  and  dyes  which  are  very  closely  re- 
lated in  composition  are  often  very  different  in  this 


COLOR  SENSITIVENESS 


77 


property.  It  is  worthy  of  note  also  that  the  par- 
ticular haloid  of  silver  used  influences  the  action  of 
the  dye  quite  materially,  some  dyes  serving  for  one 
when  they  will  not  act  with  the  others.  The  vehicle 
in  which  the  haloid  is  carried  has  also  a marked 
action;  dyes  serviceable  for  gelatine  plates  will  often 
prove  quite  useless  for  collodion. 

Why  certain  dyes  should  have  this  sensitizing 
action  on  the  silver  bromide  is  not  at  all  evident. 
Some  of  these  dyes  are  bleached  by  light,  and  one 
theory  proposed  assumes  that  the  compounds 
formed  by  this  bleaching  cause  or  help  the  reduc- 
tion of  the  silver  bromide  in  their  neighborhood. 
Against  this  explanation  is  the  fact  that  the  ease  of 
bleaching  and  the  sensitizing  action  are  not  appar- 
ently related ; dyes  which  bleach  easily  do  not  give 
the  best  or  greatest  color  sensitiveness,  and  some 
dyes  which  hardly  bleach  at  all  give  good  color 
sensitiveness.  Apparently  the  best  one  can  say  is 
that  the  silver  bromide  dye  compound,  or  complex 
as  some  writers  call  it,  acts  toward  the  other  col- 
ored lights  as  the  simple  silver  bromide  does 
toward  blue  light.  Later  we  will  consider  some  of 
the  theories  for  this  action. 

49.  Absorption  Curves.-Draper’s  law  for  the 
action  of  light  on  substances  only  needs  under- 
standing to  make  clear  its  probable  truth,  and  it 
has  also  been  proved  experimentally.  It  is  to  the 


PHOTOGRAPHY 


78 

eflfect  that  only  the  part  of  the  light  which  is  ab- 
sorbed can  produce  chemical — rather  photochemi- 
cal— action.  In  other  words,  light  which  passes 
right  through  a substance  will  not  produce  any 
change  in  it.  That  is  to  say,  if  wc  determine  what 
colors  of  light  the  dye  absorbs  when  it  is  used  to 
dye  the  silver  bromide,  these  will  be  the  colors  for 
which  it  will  sensitize  the  plate.  The  colors  the 
dye  absorbs  when  in  solution  in  water  or  in  alcohol 
are  generally  much  the  same  as  when  in  the  silver 
bromide,  so  that  if  one  lets  the  spectrum  pass 
through  a solution  of  the  dye,  the  colors  which  are 
missing  will  be  the  colors  for  which  the  dye  will 
sensitize  the  plate  if  it  changes  the  sensitiveness  at 
all.  One  places  a weak  solution  of  the  dye  in  the 
path  of  the  beam  of  light  forming  the  spectrum, 
and  then  measures  the  amount  of  each  color  which 
has  disappeared,  that  is  measures  the  absorption. 
These  values  are  then  plotted  vertically  against 
color  horizontally,  giving  an  ‘'absorption  curve’^  ^ 
which  for  a yellow-green  absorbing  dye,  for  exam- 
ple, will  resemble  Fig.  13-B.  To  the  eye  the  dye 
solution  will  appear  the  complementary  color  to  the 
ones  absorbed,  that  is  in  this  case  red. 

50.  Absorption  and  Sensitizing. — Now  let  us  dye 
an  ordinary  dry  plate  with  this  yellow-green  absorb- 

* See  also  Mees,  “Atlas  of  Absorption  Spectra.” 


COLOR  SENSITIVENESS 


79 


ing  dye,  expose  it  to  the  spectrum,  develop  it  and 
measure  the  resulting  densities.  By  plotting  these 
densities  against  color  we  will  get  a '^density-color'’ 
curve  (Fig.  13-C).  As  well  as  the  natural  color 


Fig.  13.  Curve  A is  the  color  sensitiveness  of  silver  bromide 
gelatine  emulsion.  Curve  B the  absorption  curve  of  eosine  in 
water.  Curve  C the  color  sensitiveness  of  silver  bromide  gela- 
tine emulsion  dyed  with  eosine. 


sensitiveness  maximum  in  the  blue-violet  (Fig.  13- 
A),  which  is  always  present  in  dry  plates,  we  have 
a secondary  maximum  in  the  yellow-green  due  to 
the  dye  and  corresponding  in  position  to  the  maxi- 


8o 


PHOTOGRAPHY 


mum  in  the  water-solution  absorption  curve  above 
except  that  it  is  shifted  slightly  towards  the  red 
end  of  the  spectrum.  These  color  density  curves 
are  also  color  sensitiveness  curves,  as  the  sensitive- 
ness is  approximately  proportional  to  the  density 
under  these  circumstances. 

51.  Practical  Application. — The  curves  we  get 
by  the  use  of  one  of  these  dyes  are  always  irregular 


Fig.  14.  Luminosity  curve  for  the  average  eye. 


with  peaks  and  hollows  and  not  of  the  kind  we 
would  choose  for  ideal  photography.^  If  we  wish 
to  represent  things  exactly  as  they  appear  to  the 
eye,  then  the  plate  should  have  a sensitiveness  curve 
like  the  curve  for  the  action  of  the  different  colors 
of  light  on  the  eye,  that  is  what  is  called  the  lumi- 
nosity curve;  see  Fig.  14.  While  visually  correct 
this  would  sacrifice  detail,  particularly  in  the  violets 
and  blues.  For  the  general  case  it  is  better  to  ask 
that  the  photograph  show  all  the  detail  possible  in 

® For  an  excellent  discussion  of  this  whole  subject,  see  The 
Photography  of  Colored  Objects,  by  C.  E.  K.  Mees,  or  Das 
Arbeiten  mit  Farbenemphndlicken  Flatten,  von  Dr.  Ernst 
Konig,  G.  Schmidt,  Berlin,  1909. 


COLOR  SENSITIVENESS 


8i 


all  the  different  colors.  This  requires  that  the  sen- 
sitiveness in  all  the  colors  should  be  as  nearly  alike 
as  possible,  that  is  the  color  sensitiveness  curve 


Orthochrom 


X—,  i I I L 


Fig.  15. 


82 


PHOTOGRAPHY 


should  be  a straight  line  running  horizontally 
through  all  the  colors.  This  ideal  is  approached 
fairly  closely  by  a number  of  panchromatic  plates 
now  on  the  market,  with  however  some  sacrifice  of 
plate  speed.  The  sensitiveness  curves  obtained  by 
the  use  of  certain  good  dyes^  are  given  in  Fig.  15. 
It  will  be  noted  that  the  curves  for  the  different 
dyes  are  very  far  from  the  ideal  curve  desired,  al- 
though these  dyes  are  among  the  best  known  for 
this  purpose.  The  natural  sensitiveness  in  the  blue- 
violet  exceeds  all  the  rest  and  there  are  usually  quite 
pronounced  dips  in  different  colors.  These  dips  or 
minima  occur  at  different  colors  with  the  different 
dyes  so  that  it  would  look  likely  that  one  could 
avoid  them  by  the  use  of  two  of  the  dyes  at  once, 
that  is  dye  the  plate  with  a mixture  of  two  or  more 
dyes,  or  successively  with  different  dyes.  When 
one  tries  the  experiment  this  way  it  is  soon  evident 
that  the  effect  obtained  is  not  at  all  the  sum  of  the 
effects  of  the  two  dyes  when  used  separately.  It 
becomes  necessary  to  try  the  effect  of  the  dyes  when 
used  together  by  actually  making  the  experiment. 
Such  experiments  have  shown  that  it  is  possible  to 
make  a fairly  good  plate  by  a mixture  of  certain 
dyes  in  certain  proportion  ® and  some  of  the  com- 

* Some  of  the  best  dyes  may  be  obtained  from  The  Bayer  Co., 
1 17  Hudson  St.,  New  York;  Farbwerke  Hoechst  Co.,  122  Hud- 
son St.,  New  York. 

•See  R.  J.  Wallace,  Astrophysical  Journal,  26,  317  (1907). 


COLOR  SENSITIVENESS  83 

mercial  plates  may  be  made  this  way,  as  it  is  prob- 
ably the  simplest  method.  But  the  interference  of 
one  dye  with  the  other  in  the  silver  bromide  can 
be  avoided  by  adding  one  dye  to  part  of  the  emul- 
sion and  another  dye  to  another  part  and  so  on  as 
far  as  necessary  to  build  up  the  curve  desired. 
Then  after  the  dififerent  parts  of  the  emulsion  have 
been  washed  ready  for  coating  the  plates,  they  are 
mixed  thoroughly  with  each  other;  and  the  color 
sensitiveness  of  the  resulting  plates  will  be  that 
expected  from  the  way  the  different  dyes  act  when 
alone.  In  this  way  it  is  possible  to  build  up  a sensi- 
tiveness curve  which  is  almost  straight,  the  remain- 
ing difficulty  being  the  too  great  sensitiveness  in 
the  blue-violet  and  usually  a small  dip  in  the  green 
and  in  the  deep  red.  There  are  a variety  of  names 
used  for  such  plates  by  the  manufacturers,  but  the 
term  Panchromatic  is  probably  as  good  as  any. 

52.  Screening  Action. — Besides  the  direct  sensi- 
tizing action  of  the  dye  on  the  silver  bromide  there 
is  another  action  possible  which  has  to  be  taken 
account  of ; if  the  dye  stains  gelatine  strongly  or  if 
it  is  not  well  washed  out,  then  the  surface  layers 
of  the  emulsion  will  absorb  the  light  the  dye  absorbs, 
and  this  absorbed  light  will  not  affect  the  silver 
bromide.  Usually  it  is  an  undesirable  action,  but 
it  can  be  used  to  cut  down  the  blue-violet  maximum 


84 


PHOTOGRAPHY 


by  staining  the  gelatine  with  a dye®  which  will 
absorb  some  of  the  blue-violet  light  and  so  prevent 
this  part  from  reaching  the  silver  bromide,  and  so 
reducing  this  maximum  to  any  desired  extent. 

53.  Color  Screens. — ^^-Attempts  to  build  up  the 
color  sensitiveness  to  equality  all  through  the  spec- 
trum all  meet  finally  the  difficulty  of  the  invariably 
greater  sensitiveness  of  the  silver  bromide  itself  in 
the  blue- violet.  To  get  over  this  difficulty  there 
remains  finally  only  the  objectionable  way  of  cut- 
ting down  the  amount  of  the  light  in  those  colors 
for  which  there  is  too  great  sensitiveness,  so  as  to 
leave  them  on  a footing  of  equality  with  all  the 
other  colors.  This  is  objectionable  because  it  nec- 
essarily increases  the  exposure  required,  that  is  re- 
duces the  apparent  speed  of  the  plate.  This  cutting 
down  can  be  accomplished  by  passing  the  light 
through  a transparent,  colored  material  before  it  is 
allowed  to  fall  upon  the  plate.  The  more  conven- 
ient of  these  “color  screens''  are  made  by  a layer 
of  dyed  gelatine  cemented  between  two  pieces  of 
glass  and  fastened  in  a tube  which  can  be  slipped 
over  the  front  of  the  lens  of  the  camera. 

To  accomplish  the  desired  result  it  will  be  evi- 

® Meister,  Lucius  & Briining,  Hoechst,  o.  M.,  advertise  Eryth- 
rosine — Filter  Yellow  for  subduing  the  blue  sensitiveness,  and 
the  same  dipping  bath  may  contain  pinachrom  or  pinacyanol 
for  sensitizing  to  the  other  colors.  American  agents  are  H.  A. 
Metz  & Co.,  P.  O.  Box  753,  New  York. 


COLOR  SENSITIVENESS  85 

dent  from  what  has  preceded  that  the  color  screen 
must  be  fitted  to  the  particular  plate  with  which  it 
is  to  be  used,  and  in  order  to  cut  down  only  those 
colors  desired  and  to  the  amount  desired,  the  color 
of  the  screen  must  be  very  carefully  adjusted.  A 
study  of  their  absorption  curves  will  show  what 
dye  to  use,  or  often  a mixture  of  dyes  is  needed, 
and  in  the  ordinary  case  it  will  absorb  principally 
the  blue  and  violet  light  so  that  the  screen  will 
appear  yellow  or  red.  The  adjustment  of  the 
amounts  of  the  dye  to  use  can  be  calculated  approxi- 
mately from  the  absorption  curve,  but  the  careful 
final  adjustment  of  the  amount  or  amounts,  if  more 
than  one  dye  be  used,  can  only  be  accomplished  by 
making  a screen  and  trying  it  on  the  plate."^  A very 
small  change  in  the  amounts  of  the  dyes  produce 
considerable  changes  in  the  desired  effects. 

To  consider  a particular  case  suppose  we  wish  to 
make  a color  screen  for  use  with  a plate  dyed  with 
only  one  dye,  pinachrom;  refer  to  its  color  sensi- 
tiveness curve  in  Fig.  15.  If  we  wish  to  make  the 
effect  of  the  green  and  red  light  from  the  object 
have  as  great  an  effect  on  the  plate  as  the  rest  of 
the  colors,  it  will  be  necessary  to  absorb  a large 
proportion  of  the  blue  and  violet  and  some  of  the 
orange.  The  plate  is  very  little  sensitive  to  deep 

’ R.  J.  Wallace,  “Color  Filters,”  Astrophysical  Journal,  24, 
268  (1906). 


86 


PHOTOGRAPHY 


red,  and  if  we  wish  to  hold  the  other  colors  down 
sufficiently  to  make  them  all  equal,  it  will  make  the 
exposure  extremely  long,  possibly  hundreds  of  times 
the  unscreened  plate.  This  makes  the  exposures  un- 
reasonably long,  and  a compromise  is  usually  made 
by  neglecting  the  deep  red  so  that  the  increase  in 
the  exposure  will  not  be  too  great.  Such  a plate 
as  this  is  usually  called  isochromatic  where  it  has 
very  pronounced  gaps  in  the  color  sensitiveness 
curve,  as  distinguished  from  panchromatic  where 
"^he  gaps  (or  dips)  are  small  or  absent.  A screen 
for  a good  panchromatic  plate  may  hardly  double 
the  exposure. 

It  will  be  at  once  apparent  that  any  screen  will 
not  usually  do  for  any  plate  but  they  should  be  made 
to  fit  each  other.  Since  all  plates  need  screening, 
mostly  in  the  blue-violet,  any  screen  made  for  an 
isochromatic  plate  will  help  most  other  isochromatic 
plates.  There  are  a lot  of  different  color  screens 
on  the  market  with  which  the  exposure  will  be  in- 
creased anywhere  from  twice  to  fifteen  times,  and 
it  is  a matter  of  pure  guess  how  they  will  affect 
the  picture  of  a multi-colored  object  taken  on  any 
chance  plate.  It  is  much  better  to  buy  a screen  made 
for  a particular  plate  and  use  it  with  that  plate.® 


® Eastman  Kodak  Co.  & Cramer  Dry  Plate  Co.,  St.  Louis, 
both  make  screens  adjusted  to  their  own  makes  of  plates.  They 
both  issue  excellent  booklets  on  the  general  subject. 


COLOR  SENSITIVENESS  87 

54.  Rendering  Color  Contrast. — Oolor  screens 
have  another  application  which  is  often  of  con- 
siderable value.  Suppose  we  wish  to  take  a picture 
of  an  engineering  blue  print.  If  taken  in  the  ordi- 
nary way  on  the  undyed  plate  the  blue  parts  of  the 
picture  will  be  very  nearly  as  effective  in  producing 
density  in  the  negative  as  the  white  parts,  that  is 
the  resulting  negative  will  have  very  little  contrast 
and  the  print  made  from  the  negative  will  be  so  flat 
as  to  be  almost  worthless.  This  difficulty  can  be 
avoided  by  using  a color  screen  to  absorb  the  colors 
of  light  which  are  common  to  the  white  and  blue 
parts  of  the  original,  and  making  the  negative  by 
the  use  of  the  remaining  colors  (or  part  of  them) 
which  the  blue  subject  absorbs  and  to  which  it 
therefore  appears  black.  Suppose  we  use  a plate 
which  is  sensitive  to  orange  light  and  put  a screen 
over  the  lens  through  which  only  orange  light  can 
pass,  that  is  it  is  to  absorb  all  the  other  colors. 
This  will  of  course  require  a large  increase  in  the 
exposure.  The  blue  parts  of  the  original  absorb 
orange  strongly  so  that  no  light  will  go  from  these 
parts  to  the  plate,  while  the  white  parts  will  send 
all  the  orange,  as  they  absorb  almost  none.  That 
is  there  will  be  strong  light  action  where  the  sub- 
ject is  white  and  almost  none  where  it  is  blue,  and 
the  result  will  be  a good  contrasty  negative.  The 
same  idea  can  be  applied  to  increase  contrast  be- 


88 


PHOTOGRAPHY 


tween  any  two  colors  if  a suitable  screen  ® is  to  hand 
and  it  is  particularly  useful  in  microphotography 
where  the  microscopic  specimen  is  usually  stained 
and  in  pictures  of  stained  wood  articles  to  show  the 
grains.  For  their  general  use  in  rendering  clouds 
and  distance  see  Art.  132. 

® Wratten  & Wainwright,  Croyden,  England,  and  Eastman 
Kodak  Co.,  Rochester,  N.  Y.,  make  a full  line  of  such  screens. 


CHAPTER  IV 


LATENT  IMAGE  THEORIES 

55.  Basic  Facts. — Before  describing  any  of  these 
theories  it  will  be  well  to  review  the  important  facts 
about  the  latent  image.  If  light  be  allowed  to  act 
for  a short  time  on  a sensitive  film,  it  produces  no 
visible  change,  but  it  does  produce  a change  which 
makes  the  film  act  differently  towards  some  rea- 
gents. Reducing  agents  act  more  readily  on  the  ex- 
posed parts  while  the  same  parts  fix  more  slowly- 
in  hypo.  This  state  which  shows  these  differences 
in  the  different  parts  of  the  film  we  call  the  ^latent 
image.'' 

This  latent  image  may  be  destroyed  by  a number 
of  reagents,  such  as  nitric  acid,  free  halogens  or 
bodies  which  readily  give  up  halogen  like  ferric 
chloride,  FeCh,  cupric  chloride,  CuCh,  mercuric 
chloride,  HgCh,  potassium  iodide,  KI,  and  the  hal- 
ogen acids,  HCl,  HBr,  and  HI,  and  most  strong 
oxidizing  agents,  such  as  potassium  persulphate, 
K2S2O8,  and  potassium  permanganate,  KMn04  in 
acid  solution. 

If  the  film  carrying  the  image  be  allowed  to  stand 

89 


90 


PHOTOGRAPHY 


the  image  will  gradually  fade  out,  the  time  required 
varying  very  much  for  the  different  kinds  of  sensi- 
tive films.  A great  many  things  retard  or  hasten 
the  process;  some  papers  which  have  a lot  of  chro- 
mate in  the  film  to  harden  it  will  fade  out  in  a few 
days.  Most  good  plates  keep  the  image  a very  long 
time,  and  instances  are  on  record  where  good  nega- 
tives have  been  developed  years  after  exposure. 
The  action  of  the  light  on  the  film  does  not  begin 
with  the  first  light  striking  it,  but  the  action  gradu- 
ally builds  up  as  more  light  is  added.  This  starting 
action  quickly  fades  out  if  only  a small  quantity  of 
light  is  allowed  to  act,  and  this  light  producing 
small  action  has  to  be  added  again  in  the  next  ex- 
posure. After  a normal  exposure  the  formation  of 
the  latent  image  does  not  cease  with  the  stopping  of 
the  light  addition  but  continues  for  some  time,  min- 
utes perhaps,  and  then  the  gradual  fading  out 
begins. 

It  is  possible  to  make  an  artificial  latent  image 
by  allowing  a dilute  developer  to  act  on  a plate,  and 
also  by  heating  the  plate  somewhere  above  ioo°  C. 

An  enormous  amount  of  experimental  work  ^ has 
been  done  on  the  latent  image  and  its  properties, 
and  some  of  these  results  will  be  referred  to  later 
on,  but  till  the  theories  have  been  described  the 

^ Sheppard  & Mees,  “Investigations/'  Chap.  VI. ; Bancroft, 
“The  Latent  Image/'  Jour,  of  Phys.  Chem.,  17,  93  (1913). 


LATENT  IMAGE  THEORIES  91 

above  will  be  the  main  facts  which  the  theory  must 
account  for. 

56.  Theories. — There  are  a considerable  number 
of  such  theories,  but  for  our  purpose  we  will  refer 
only  to  four.  They  are  that  the  latent  image  con- 
sists of : 

1 A physical  modification  of  the  silver  salt. 

2 Small  discrete  particles  of  silver  (silver  grain  hy- 
pothesis). 

3 Silver  sub-bromide  in  the  silver  bromide  particles. 

4 A solid  solution  of  silver  in  silver  bromide  particles. 

57*  The  first  theory  is  that  the  action  of  the 
light  is  to  produce  some  physical  change  in  the  sil- 
ver bromide,  such  as  for  example  shattering  the 
particles  and  the  fragments  would  probably  be  more 
easily  developed  than  the  unaffected  particles  (com- 
pare pressure  marks).  Or  it  may  be  to  polymerize 
the  silver  bromide  or  to  unpolymerize  (that  is  to 
divide  the  molecule  into  smaller  ones,  unpolymerize, 
or  to  gather  several  molecules  into  a larger  one, 
polymerize),  or  some  writers  do  not  attempt  to  say 
what  the  nature  of  the  physical  change  is.  Then 
assuming  that  the  change  makes  the  particle  more 
easily  developed,  the  theory  will  account  for  the 
latent  image  as  far  as  its  developing  properties  are 
concerned.  But  this  theory  fails  completely  to  ac- 
count for  the  destruction  of  the  latent  image  by  all 
the  chemical  reagents  given  above ; there  is  no  sim- 


92 


PHOTOGRAPHY 


pie  way  in  which  the  reagent  can  be  made  to  gather 
the  shattered  particles  together  again.  The  theory 
is  not  very  generally  held  but  deserves  mention  for 
its  place  in  history  and  for  its  simplicity. 

58.  The  silver  grain  hypothesis  begins  with  the 
experimental  fact  that  light  separates  some  of  the 
bromine  from  the  silver,  so  that  there  are  left  small 
particles  of  free  silver  scattered  through  the  gela- 
tine but  not  necessarily  in  contact  with  the  remain- 
ing silver  bromide.  When  the  developer  comes 
along  it  is  assumed  to  carry  silver  from  the  silver 
bromide  grains  in  the  neighborhood  and  deposit  it 
on  the  silver  grain.  This  assumption  had  as  justifi- 
cation Eder’s  experiment,  where  he  touched  a plate 
in  the  developer  with  a silver  wire  and  there  was  a 
deposit  of  silver  at  the  point  of  contact  with  the 
film.  It  was  soon  found  that  the  silver  wire  had 
to  press  on  the  film — compare  pressure  marks — 
finely  divided  silver  lying  on  the  film  would  not 
produce  the  deposit.  But  nevertheless  it  is  an  ex- 
perimental fact,  silver  particles  will  grow  in  size  in 
the  film  in  the  presence  of  the  developer  and  more 
quickly  if  there  is  some  silver  nitrate  added  to  the 
developer.  But  without  the  presence  of  the  silver 
nitrate  the  growth  is  too  small  to  be  of  any  use 
practically  and  will  not  account  for  the  growth  of 
the  silver  image.  Even  if  finely  divided  silver  is 
actually  incorporated  in  the  film  itself  by  being 


LATENT  IMAGE  THEORIES  93 

mixed  with  the  molten  emulsion,  it  will  not  produce 
appreciable  development  unless  the  silver  particles 
are  actually  in  contact  with  the  silver  bromide  par- 
ticles or  actually  incorporated  in  them;  in  which 
latter  case  the  film  develops  vigorously.  But  this 
case  is  the  theory  4,  page  91.  Other  things  also  tell 
against  the  silver  grain  theory,  as  for  example  the 
fact  that  the  latent  image  is  not  destroyed  by  dilute 
nitric  acid  whereas  finely  divided  silver  is  very  read- 
ily dissolved  by  it.  Other  silver  solvents  act  simi- 
larly. So  that  this  hypothesis  has  had  to  be  modified 
to  give  the  fourth  theory  above. 

59*  The  silver  sub-bromide  hypothesis  (theory 
3,  page  91 ) is  one  which  has  held  the  field  for  a long 
time  and  possibly  is  the  most  popular  theory  yet. 
It  is  also  based  on  the  experimental  fact  of  the 
separation  by  light  of  some  bromine  from  the  silver 
bromide,  but  it  assumes  that  this  takes  place  right 
in  or  on  the  silver  bromide  particle  and  that  it 
leaves  a compound  of  silver  with  bromine  which 
therefore  is  richer  in  silver  than  the  ordinary  silver 
bromide.  The  composition  of  this  compound  is 
quite  unknown,  but  it  is  often  spoken  of  as  if  it  were 
Ag2Br,  for  which  there  is  no  real  justification.  Cary 
Lea  made  a lot  of  substances  resembling  the  latent 
image  in  properties  and  which  contain  less  bromine 
than  the  ordinary  bromide.  They  were  made  usu- 
ally by  taking  some  of  the  bromine  from  silver 


94 


PHOTOGRAPHY 


bromide,  that  is  reducing  it  by  substances  like  de- 
velopers. The  compounds  are  variously  and  often 
beautifully  colored;  dilute  nitric  acid  will  not  take 
the  extra  silver  out  of  them  nor  will  hypo  dissolve 
them  completely,  but  the  latter  always  leaves  a resi- 
due of  metallic  silver.  Other  experiments  have 
shown  that  some  of  the  metals  are  soluble  to  some 
extent  in  their  fused  chlorides.  Addition  of  a free 
halogen  to  the  latent  image  destroys  it  if  suitable 
precautions  are  taken  to  prevent  the  halogen  being 
all  taken  up  by  the  gelatine.  When  a plate  is  black- 
ened by  light  free  halogen  can  be  detected  by  the 
odor;  even  a short  exposure  to  light  produces  a 
visible  change  in  the  color  of  the  plate  even  though 
no  bromine  seems  to  be  given  off.  Probably  the 
bromine  is  taken  up  first  by  the  gelatine  till  pres- 
ently it  becomes  saturated.  All  of  these  facts  can 
only  mean  that  there  will  be  an  excess  of  silver  in 
the  light  affected  haloid,  and  experiment  has  shown 
that  this  sort  of  a haloid  is  easily  reduced.  Whether 
this  excess  of  silver  is  present  in  the  form  of  sub- 
bromide or  in  the  form  of  very  finely  divided  metal- 
lic silver  suspended  in  the  rest  of  the  silver  bromide 
(that  is  in  colloidal  solution),  is  a point  very  hard 
to  determine  directly,  but  the  indirect  evidence 
favors  the  free  silver  hypothesis. 

6o.  The  fourth  hypothesis  is  the  solid  solution 
theory,  which  is  referred  to  in  the  last  line  of  the 


LATENT  IMAGE  THEORIES 


95 


preceding  paragraph.  To  repeat,  the  latent  image 
consists  of  very  finely  divided  metallic  silver  sus- 
pended in  the  solid  silver  bromide  much  as  fine 
particles  would  be  suspended  in  water.  Such  sus- 
pensions are  now  called  ^'colloidal  solutions.’’  More- 
over, solutions  do  not  need  to  be  liquid ; many  cases 
are  known  where  a solid  dissolves  other  substances, 
and  these  cases  are  usually  referred  to  as  ''solid 
solutions,”  so  that  in  the  case  of  the  silver  in  the 
silver  bromide  it  will  be  a "colloidal  solid  solution.” 
In  this  connection  it  has  been  shown  experimentally 
that  silver  bromide  precipitated  from  solution  con- 
taining also  suspended  materials  carries  down  these 
suspended  materials  imbedded  in  the  precipitate. 
This  is  not  limited  to  suspended  solids  but  takes 
place  even  with  substances  which  are  in  true 
solution.  These  carried-down  materials  may  be 
the  dyes  that  we  have  been  studying  a few  pages 
back,  or  other  metallic  chlorides,  or  gelatine,  or 
albumen,  or  a great  many  other  substances.  They 
are  carried  down  even  though  they  are  present  in 
very  small  amount  and  it  is  almost  impossible  to 
wash  them  out  completely  from  the  precipitate. 

The  presence  of  these  small  quantities  of  inclu- 
sions may  affect  the  sensitiveness  of  the  plate  very 
materially,  as  in  the  case  of  the  dyes,  or  in  the  case 
of  mercuric  chloride  or  of  ferric  chloride,  in  which 
latter  cases  the  sensitiveness  will  be  greatly  reduced. 


PHOTOGRAPHY 


96 

The  presence  of  0.001  gram  of  gelatine  in  10,000 
cc  of  solution  from  which  the  silver  salt  was  pre- 
cipitated distinctly  increased  the  sensitivity.  The 
amount  of  silver  actually  set  free  by  the  light  and 
thus  forming  the  latent  image  must  be  very  minute, 
but  all  these  facts  go  to  show  that  it  may  be  quite 
sufficient  to  accomplish  the  result  of  making  the 
silver  haloid  more  easily  reduced.  Finely  divided 
silver  may  be  carried  down  in  the  same  way  by  the 
precipitated  silver  haloid.  The  color  of  such  finely 
divided  silver  varies  with  the  state  of  division  of 
the  silver  from  white  to  pink,  rose  colored,  red, 
brown,  blue,  to  black.  The  color  of  the  silver  haloid 
precipitate  carrying  the  finely  divided  silver  agrees 
exactly  with  it.  Treatment  with  strong  nitric  acid 
does  not  take  out  the  extra  silver  unless  the  treat- 
ment is  very  prolonged — 25  hours  at  100°  C in  one 
case.  Almost  any  means  of  producing  the  haloid, 
and  at  the  same  time  reducing  some  of  it  to  metallic 
silver,  will  give  these  colored  precipitates.  The  col- 
ored chloride  precipitate  dissolves  very  slowly  in 
ammonia  solution,  leaving  a residue  of  metallic  sil- 
ver which  may  amount  in  extreme  cases  to  7% 
of  the  haloid  but  is  usually  around  1%.  Hypo  or 
ammonium  chloride  solutions  will  do  the  same.  On 
exposure  to  light  all  these  colored  haloids  blacken 
very  rapidly,  more  rapidly  than  the  uncolored. 
They  are  all  very  readily  reduced  to  metallic  sil- 


LATENT  IMAGE  THEORIES 


97 


ver  by  developers.  In  fact,  the  behavior  of  these 
compounds  is  exactly  like  that  of  the  latent  image. 

If  an  exposed  plate  be  fixed  in  plain  hypo  it  is  still 
possible  to  get  a negative  from  it  by  developing  it 
in  a solution  which  will  deposit  metallic  silver.  Plain 
hypo  solution  does  not  dissolve  metallic  silver  even 
when  the  latter  is  finely  divided,  but  if  acid  bisul- 
phite be  added  to  the  plain  hypo  solution,  the  latent 
image  will  be  destroyed,  that  is  it  can  no  longer  be 
developed  by  a solution  which  deposits  silver,  be- 
cause the  acid  bisulphite  dissolves  the  silver  form- 
ing the  latent  image  at  the  same  time  that  the  hypo 
is  dissolving  the  rest  of  the  silver  haloid.  The 
silver  particles  forming  the  latent  image,  being 
bedded  in  the  silver  haloid,  are  protected  by  it  from 
such  reagents  as  nitric  acid,  and  the  presence  of 
the  silver  particles  makes  the  particle  of  haloid  less 
soluble  in  its  solvents  but  much  more  readily  re- 
duced by  developers. 

6i.  Conclusion. — From  all  this  work  it  will  ap- 
pear that  the  hypothesis  of  the  formation  of  silver 
sub-bromide  is  quite  unnecessary  to  explain  the  ex- 
perimental facts,  the  colloidal  solution  theory,  being 
simpler,  is  to  be  preferred.  Also  in  a number  of 
cases  the  latter  hypothesis  is  in  better  agreement 
with  the  experimental  facts,  as  for  example  in  the 
colors  of  Cary  Lea’s  photohaloids.  It  seems  best 
to  consider  the  latent  image  as  consisting  of  silver 


PHOTOGRAPHY 


98 

particles  suspended  in  silver  bromide  in  colloidal 
solution — that  is  in  a very  fine  state  of  division. 
The  action  of  the  light  has  been  to  set  free  some 
bromine  which  combines  with  the  gelatine,  leaving 
a minute  portion  of  metallic  silver  incorporated  in 
the  particle  of  silver  haloid,  where  its  mere  pres- 
ence greatly  increases  the  ease  of  reduction. 


CHAPTER  Vi 


NEGATIVE  DEFECTS 

62.  The  following  series  of  defects  in  negatives 
are  of  frequent  enough  occurrence  to  require  a 
somewhat  extended  discussion. 

1.  Thin  from  (a)  Lack  of  sufficient  silver  bromide  on  the 

plate 

(b)  Underexposure 

(c)  U nderdevelopment— Intensification 

2.  Dense  from  (a)  Overexposure 

(b)  Overdevelopment — ^Reduction 

(c)  Fog,  from  light,  from  age,  or  from 
defective  emulsion. 

3.  Frilling, 

4.  Dust  marks, 

5.  Air  bells, 

6.  Black  spots, 

7.  Finger  marks, 

8.  Pressure  marks, 

9.  Halation, 

10.  Oyster  shell  markings, 

11.  Stain, 

12.  Bubbles  in  the  gelatine, 

13.  Drying  troubles. 

63.  Thin  Negatives. — The  negative  may  lack 
sufficient  printing  density  so  that  detail  visible  in 
the  negative  will  not  show  in  the  print.  This  may 
be  due  to  a number  of  causes.  In  the  first  place 

99 


100 


PHOTOGRAPHY 


the  total  silver  bromide  put  on  the  plate  in  the  emul- 
sion by  the  maker  may  not  have  been  sufficient; 
that  is  there  may  not  have  been  sufficient  of  the 
emulsion  or  the  emulsion  may  have  been  too  poor 
in  silver  bromide.  The  remedy  is  to  make  another 
negative  on  another  make  of  plate.  But  it  rarely 
happens  with  reliable  makers  of  plates  and  usually 
requires  direct  experiment  to  show  that  it  is  the 
cause  of  the  thinness.  Nevertheless  if  one  has  a 
series  of  thin  negatives  of  which  the  exposure  and 
development  can  be  relied  upon,  then  it  will  be  worth 
while  to  try  another  make  of  plate  to  see  whether  it 
will  help  the  trouble. 

Or  it  may  be  underexposed,  in  which  case  the 
shadows  will  lack  detail  and  the  print  will  be  flat. 
It  may  even  have  what  resembles  a white  veil  all 
over  the  print,  which  one  sees  very  frequently  in 
''snap  shots.’'  Occasionally  one  can  build  up  suffi- 
cient printing  density,  so  that  the  print  becomes 
clear,  but  the  intensification  will  not  find  out  any 
more  detail  than  there  is  already  there.  As  well  as 
lacking  detail  the  resulting  print,  after  the  intensifi- 
cation, is  sure  to  be  harsh  and  without  half-tones, 
in  general  unsatisfactory.  If  the  underexposure  is 
at  all  serious  the  only  real  remedy  is  another  nega- 
tive giving  more  exposure,  2,  4,  or  8 times  as  much ; 
there  is  seldom  any  use  in  changing  the  exposure  in 
less  than  factors  of  two. 


NEGATIVE  DEFECTS 


lOI 


Or  the  negative  may  be  underdeveloped,  due  to 
too  short  a time  in  the  developer,  to 'too  weak,  or 
too  cold  a developer.  In  this  case  there  will  be 
more  detail  visible  in  the  negative  generally,  and 
particularly  in  the  shadows.  Intensification  will 
set  the  case  right  exactly  as  the  process  simply  sup- 
plements, or  rather  continues,  the  incomplete  proc- 
ess of  development.  In  this  case,  a satisfactory  in- 
tensification is  a complete  remedy,  but  in  the  other 
cases  it  is  only  a palliative,  though  it  will  often  im- 
prove matters. 

That  is  to  say,  in  the  great  majority  of  cases 
both  intensification  and  reduction  are  attempts  to 
patch  up  previous  faulty  work.  In  most  cases  the 
best  remedy  is  to  repeat  the  work,  correcting  the 
fault.  In  case  it  is  not  possible  to  repeat  the  ex- 
posure or  in  case  the  subject  is  a difficult  one,  these 
methods  may  be  the  only  available  resource. 

64.  Intensification. — A very  great  many  proc- 
esses of  intensification  ^ have  been  described,  any 
one  of  which  will  undoubtedly  add  density  to  the 
silver  image.  With  many  of  them  staining  is  diffi- 
cult to  avoid.  With  most  of  them  the  intensifica- 
tion is  very  irregular  both  on  different  parts  of  the 
same  plate  and  on  different  plates.  The  added 
density  is  not  proportional  to  the  original  density 

^ For  example  see  B.  J.  Almanac  for  1915,  p.  583,  or  Photo- 
miniature, No.  74. 


102 


PHOTOGRAPHY 


nor  does  it  vary  from  the  proportionality  in  any 
definite  way,  so  that  the  gradation  in  the  resulting 
negative  is  largely  a matter  of  chance.  In  some 
cases  this  is  due  to  the  intensifier  actually  reducing 
part  of  the  image  while  intensifying  the  rest.  Many 
workers  recommend  the  use  of  such  intensifiers  ^ 
(and  reducers)  as  a means  of  controlling  the  grada- 
tion in  the  negative.  For  all  record  photography 
such  manipulation  is  to  be  condemned,  but  in  pic- 
torial photography  it  has  a place  as  giving  the 
worker  control,  even  if  uncertain,  over  the  charac- 
ter of  the  picture  which  in  such  work  has  no  neces- 
sary relation  to  the  original  subject. 

A method  which  may  be  relied  upon  to  give  pro- 
portional intensification  is  the  one  advocated  by 
Chapman  Jones  ^ and  described  in  detail  in  the 
Manual,  Exp.  lo;  see  also  Art.  24.  The  density  is 
almost  exactly  doubled  by  one  application  and  the 
gradation  is  not  altered.  After  the  final  washing 
the  process  may  be  repeated  with  similar  results. 
In  negatives  where  gradation  is  not  important,  as 
in  line  drawings  and  where  great  intensification  is 
desired,  the  bleached,  well-washed  negative  may  be 
immersed  in  a dilute  solution  of  sodium  sulphide, 

2 See,  for  example,  W.  F.  Ellis,  American  Photography,  8, 
293  (1914). 

“ See  Photominiature,  No.  74,  page  67,  or  also  British  Journal, 
57>  495  (1910)- 


NEGATIVE  DEFECTS 


103 

by  which  both  the  silver  and  the  mercury  chlorides 
will  be  changed  to  brown  sulphides,  thus : 

2AgCl+Na2S  = Ag2S+2NaCl 

2HgCl+Na2S  = Hg2S+ 2NaCl 

and  the  density  of  the  negative  will  be  very  greatly 
increased,  but  the  process  cannot  now  be  repeated. 

65.  Overexposure. — Too  dense  a negative  may 
also  be  due  to  several  causes.  The  negative  may 
have  been  exposed  much  longer  than  was  necessary 
and  yet  hardly  be  what  should  be  called  overex- 
posed. That  is  the  densities  may  still  lie  on  the 
straight  part  of  the  characteristic  curve.  The  prints 
made  from  it  will  be  correct  in  rendering  of  the  light 
and  shade,  but  the  time  of  printing  will  be  long.  The 
safest  remedy  is  to  spend  the  time  required  for  the 
printing.  If  it  can  be  arranged  to  use  daylight,  or  in 
extreme  cases  sunlight,  the  exposure  can  readily  be 
made  as  short  as  will  be  manageable.  If,  however, 
the  exposure  has  been  so  great  that  the  densities 
have  got  beyond  the  straight  part  of  the  curve  to  any 
extent,  the  plate  will  be  lacking  in  contrast  and  look 
fogged  all  over.  The  printing  qualities  may  be  im- 
proved by  a complicated  process  of  reduction  and 
intensification,  requiring  considerable  skill  and 
familiarity  with  these  processes,  and  it  is  only  in 
rare  cases  that  it  is  profitable.  Ordinarily  the 
print  will  be  flat,  that  is  lacking  in  contrast.  In 


104 


PHOTOGRAPHY 


such  cases,  the  simplest,  quickest,  and  best  course 
is  to  take  another  negative  with  a very  much  shorter 
exposure,  ^ or  % or  even  less,  depending  on  the 
degree  of  overexposure. 

66.  Overdevelopment. — The  great  density  may 
have  been  due  to  too  long  in  the  developer,  to  too 
concentrated  a developer,  or  to  too  warm  solution, 
any  one  of  which  will  carry  the  development  too 
far,  that  is  give  too  large  a development  factor, 
which  will  mean  the  exaggeration  of  the  contrasts 
and  the  increase  of  any  halation  which  may  have 
been  present.  The  halation  deposit  being  mostly  on 
the  under  side  of  the  film,  it  is  practically  impos- 
sible to  remove  it  without  at  the  same  time  destroy- 
ing the  negative  detail.  If  the  plate  is  important 
and  another  exposure  too  difficult  to  get,  the  best 
plan  is  to  spend  the  time  necessary  to  make  the 
exposure,  shading  the  plate  locally  so  as  to  force 
the  exposure  at  the  dense  places.  A few  trials  will 
often  enable  one  to  make  quite  satisfactory  prints. 
Lacking  any  great  amount  of  halation,  the  best  rem- 
edy is  one  of  the  reducers. 

67.  Fog. — The  large  total  density  may  be  at 
least  partly  due  to  fog.  This  has  been  referred  to 
already  in  Arts.  19,  34  and  37.  It  is  a deposit  of 
silver  in  the  film  of  the  negative  which  is  not  the 
result  of  the  action  of  the  light  used  to  make  the 
negative  exposure.  Overdevelopment  is  liable  to 


NEGATIVE  DEFECTS  105 

include  as  a result  a lot  of  chemical  fog.  If  this 
is  even  over  the  whole  plate  it  will  merely  mean 
longer  printing  exposure,  but  it  will  not  be  even,  as 
can  readily  be  seen.^  The  amount  of  fog  formed  at 
any  given  place  in  a given  time  depends  on  the 
amount  of  unreduced  silver  haloid  at  that  place. 
From  which  it  follows  that  the  fog  will  be  greatest 
at  the  places  of  least  density  in  the  negative.  Car- 
ried to  the  extreme,  this  will  mean  that  the  nega- 
tive will  get  black  all  over,  which  actually  happens 
if  it  is  kept  long  enough  in  the  developer.  The 
effect  of  the  greater  fog  in  the  thin  places  will  be 
to  reduce  the  contrast  in  the  negative,  so  that  while 
normally  the  effect  of  long  development  is  to  in- 
crease contrasts,  this  increase  of  contrasts  may  be 
partially  or  completely  offset  by  the  formation  of 
fog.  Reduction  will  help  materially  if  the  right 
reducer  be  used,  that  is  one  which  will  dig  out  the 
thin  parts  more  proportionately  than  the  thicker, 
which  will  tend  to  undo  the  fogging.  The  fog  is 
liable  to  be  a good  deal  on  the  surface,  which  will 
help  its  removal  by  the  reducer.  Besides  chemical 
fog  referred  to  above  another  common  cause  is 
‘light  struck,’’  due  to  some  unpremeditated  expo- 
sure of  the  plate  to  light.  The  camera  or  plate 
holders  may  not  be  tight  (if  possible  never  let  the 

‘Lumiere  & Seyewetz,  Brit.  Jour.,  54,  195  (1907)  ; Sheppard 
& Mees,  Investigations,  page  59. 


io6 


PHOTOGRAPHY 


sun  shine  directly  on  the  plate  holders),  the  dark 
room  light  may  not  be  safe,  or  the  dark  room  may 
not  be  really  dark.  In  case  it  is  even  over  the 
whole  plate,  the  safest  procedure  is  to  spend  the  re- 
quired longer  time  over  the  printing,  as  besides  the 
risk  of  mishap  reduction  will  often  change  the  con- 
trasts so  as  to  impair  the  printing  quality.  In  case 
the  light  struck  parts  are  not  evenly  distributed,  as 
is  usually  the  case,  attempts  at  reduction  usually 
do  not  improve  matters;  the  safest  procedure  is  to 
manipulate  the  printing  exposure  by  shading  part 
of  the  print  with  the  hands  or  cut-out  cardboard 
so  that  the  denser  parts  will  receive  the  required 
extra  exposure.  It  will  take  a number  of  trials  to 
get  the  right  proportion  between  the  various  expo- 
sures when  satisfactory  prints  can  often  be  made. 
In  case  a large  number  of  prints  are  desired  it  will 
pay  to  make  a new  negative  from  one  of  the  best 
obtainable  prints  and  from  which  good  prints  can 
be  readily  made. 

Fog  may  be  due  to  the  plates  being  old,  or  having 
been  kept  in  damp  or  hot  places.  The  edges  of  such 
plates  usually  show  it  more  than  the  middle;  some- 
times the  edges  are  quite  thin,  due  to  the  falling  off 
in  sensitiveness  of  the  film.  Plates  go  bad  very 
quickly  when  kept  in  damp  places. 

From  the  discussion  of  the  manufacture  of  dry 
plates  you  will  understand  that  the  plates  can  be 


NEGATIVE  DEFECTS 


107 


badly  made  so  that  they  fog  heavily  in  the  devel- 
oper. But  with  reliable  makers  this  rarely  happens. 

68.  Reduction. — There  are  a great  many  re- 
ducers which  one  might  use,  just  as  there  are  a 
great  many  intensifiers.  They  can  be  divided  into 
three  general  classes. 

(a)  Those  which  increase  the  density  ratios  by 
removing  more  silver  proportionally  from  the  thin 
parts  of  the  image  than  from  the  denser  parts.  An 
example  of  this  class  is  the  ferricyanide-hypo  re- 
ducer (also  called  Howard  Farmer  reducer).  See 
Manual,  Exp.  ii. 

(b)  Those  which  do  not  alter  the  density  ratios, 
that  is,  remove  proportionate  amounts  of  silver 
from  all  parts  of  the  image.  The  potassium  per- 
manganate reducer  comes  near  to  this  though  really 
belonging  in  class  (a).^ 

(c)  Those  which  reduce  the  density  ratios  by  re- 
moving more  proportionately  from  the  denser  parts 
of  the  image.  The  best  example  is  the  ammonium 
persulphate  reducer. 

The  microscope  ® shows  that  class  (a)  sinks  into 
the  film,  dissolving  practically  all  the  silver  particles 
as  it  goes  in.  The  microscope  also  shows  that  the 
silver  is  distributed  deeper  into  the  film  in  the 

*Huse  & Nietz,  Scientific  American  Supplement,  Dec.  23,  p. 
405  (1916). 

•Dr.  W.  Scheffer,  Brit,  Jour.  54,  540  (1907),  and  55,  472 
(1908). 


io8  PHOTOGRAPHY 

denser  parts,  less  deep  in  the  thinner  parts.  The 
reducer  mentioned  then  acts  just  as  if  one  took  a 
shaving  off  the  surface  of  the  film,  which  might 
easily  remove  all  the  deposit  from  the  thin  places 
while  leaving  untouched  a good  share  of  that  in 
the  denser  places,  that  is,  of  course,  changing  the 
proportions  among  the  densities. 

The  ammonium  persulphate  has  a very  peculiar 
action.  The  microscope  shows  that  it  attacks  the 
denser  parts  of  the  image  strongly,  while  the  thin 
parts  are  almost  proof  against  it,  with  the  results 
that  it  cuts  down  the  contrasts  as  much  as  desired. 
Both  the  solid  ammonium  persulphate  and  the  solu- 
tion in  water  (3%)  used  as  the  reducer  are  very 
unstable,  decomposing  steadily  so  that  the  more 
stable  one,  the  solid,  will  keep  only  a few  months. 

The  chemical  action  of  all  these  reducers  is  a 
good  deal  alike.  They  change  the  silver  from  metal 
to  a metallic  salt  which  is  soluble  in  water  or  in 
some  other  reagent  and  is  thus  removed  from  the 
film.  Thus  for  the  ferricyanide 

2Ag-f  2K3Fe(CN)6  = K4Fe(CN)6+K2Ag2Fe(CN)6 

The  resulting  potassium  ferrocyanide  is  soluble  in 
water  and  the  potassium  silver  ferrocyanide  dis- 
solves in  the  hypo  thus 

K2Ag2Fe(CN)6+3Na2S203  = Na2K2Fe(CN)e+Na4Ag2(S203)3 


NEGATIVE  DEFECTS  109 

And  for  the  persulphate  reducer  thus, 

(NH4)2S20s+2Ag  = 2AgNH4S04  = Ag2S04+(NH4)2S04 

and  both  silver  and  ammonium  sulphates  formed 
are  soluble  in  water. 

69.  Local  Reduction. — At  times  it  may  be  ad- 
vantageous to  reduce  part  of  a plate  and  leave  the 
rest  unchanged.  This  can  be  done  by  applying  one 
of  the  above  reducers  with  a brush  where  it  is  de- 
sired to  reduce  the  deposit,  and  rinsing  the  plate 
frequently  in  water  to  stop  the  reducer  spreading 
and  to  avoid  m^arks.  It  is  not  possible  to  limit  the 
reduction  at  a sharp  line  but  it  can  be  made  to 
give  nicely  graded  effects.  Fairly  sharp  line  limits 
to  the  reduction  may  be  obtained  by  rubbing  the 
dry  film  with  a cloth  wet  with  absolute  alcohol, 
which  does  not  soften  gelatine.  The  surface  of 
the  film  can  be  rubbed  off,  carrying  with  it  the  silver 
deposit  and  so  reducing  the  density  locally.  This 
may  also  be  accomplished  by  grinding  off  the  sur- 
face of  the  dry  film  with  an  abrasive  powder  or 
by  scraping  it  off  with  a sharp  knife.  It  is  very 
difficult  to  do  any  of  these  things  so  that  they  will 
not  be  seen  easily  in  the  print. 

70*  In  normal  exposures  developed  without  the 
presence  of  added  bromide,  the  proportions  between 
the  densities,  that  is  the  gradation,  was  not  under 
any  control,  and  the  negative  remained  always  true 


no 


PHOTOGRAPHY 


to  the  subject.  The  magnitude  of  the  density  and 
with  it  the  contrast,  was  all  that  could  be  altered. 
But  by  the  use  of  these  intensifiers  (except  the 
Chapman  Jones)  and  reducers  without  exception, 
these  proportions,  the  gradation,  may  be  manipulat- 
ed very  greatly,  and  the  picture  will  be  no  longer  a 
true  record  of  the  light  and  shade.  One  used  to 
studying  photographs  will  frequently  detect  it. 
Their  use  is  in  helping  already  very  defective  nega- 
tives, which  however  they  do  not  make  true,  and  in 
pictorial  work.  They  have  always  to  be  used  with 
great  judgment  or  they  will  make  matters  worse. 
Their  indiscriminate  use  is  to  be  heartily  con- 
demned. 

71.  Frilling. — ^When  dry  the  gelatine  layer  ad- 
heres to  clean  glass  with  very  great  tenacity.  As 
the  film  dries  it  shrinks  so  that  there  is  usually 
tension  between  the  film  and  the  glass.  If  the  film 
becomes  exceedingly  dry,  as  it  does  occasionally  in 
our  steam-heated  rooms  in  the  winter,  the  tension 
becomes  so  great  that  something  breaks.  Quite  fre- 
quently the  film  pulls  the  glass  off  in  small  chips 
which  will  be  found  adhering  to  the  gelatine  film. 
When  water  soaked,  however,  the  union  between  the 
glass  and  the  film  is  weak  enough  to  at  times  give 
trouble  from  the  two  parting  company.  It  is  called 
frilling  and  commonly  takes  place  around  the  edges 
of  the  plate,  but  in  serious  cases  may  result  in  loos- 


NEGATIVE  DEFECTS 


III 


ening  the  entire  film  from  the  glass.  It  is  much 
more  troublesome  in  summer  than  in  winter,  and 
much  more  in  warm  climates  than  in  the  cooler 
ones.  It  can  usually  be  controlled  by  using  cool 
solutions  and  wash  water,  below  22^  C.  (75° 
F.)  if  possible.  Some  of  the  manufacturers  have 
special  plates  for  hot  climates  in  which  the  gelatine 
used  is  harder,  that  is  has  a higher  melting  point 
and  consequently  does  not  soften  so  much  in  the 
warm  water.  The  purpose  of  the  alum  which  we 
use  regularly  in  the  hypo  solution  is  to  harden  the 
gelatine,  so  avoiding  frilling  in  the  later  washing 
and  also  in  the  hypo  itself.  In  case  one  uses  plain 
hypo  it  will  often  be  necessary  to  put  the  plate  into 
a bath  of  alum  after  the  developer,  so  that  it  will 
not  frill  too  badly  in  the  fixing  and  washing. 

72.  Dust. — Sometimes  on  looking  over  a nega- 
tive you  will  find  a lot  of  small  clear  places  of 
irregular  shape  and  sharp  angles  which  will  come 
out  as  black  spots  in  the  print.  These  are  due  to 
dust  on  the  plate  during  exposure.  In  the  original 
box  the  plates  are  apt  to  be  free  of  dust,  and  if 
the  plate  holder  be  kept  thoroughly  clean  there  is 
not  likely  to  be  trouble.  Besides  seeing  that  the 
plate  holder  is  clean,  it  is  a wise  precaution  to  dust 
the  plate  very  lightly  with  a lintless  duster  or  cam- 
eks-hair  brush.  If  the  plate  be  rubbed  vigorously 
the  film  surface  will  become  electrified  and  attract 


II2 


PHOTOGRAPHY 


dust  strongly — it  will  be  worse  than  if  the  plate  was 
left  alone.  An  alternative  plan  is  to.  blow  gently 
along  the  film,  being  very  particular  not  to  blow 
any  saliva  onto  the  film,  as  it  will  leave  a spot. 

73o  Air  Bells. — Sometimes  one  will  come  on 
round  clear  spots,  larger  than  dust  marks,  or  the 
spots  may  be  irregular  with  gently  curved  edges. 
These  are  usually  due  to  air  bubbles  sticking  to 
the  film  during  all  or  a part  of  development.  Run- 
ning the  tips  of  the  fingers  or  a soft  brush  over  the 
film  right  at  the  commencement  of  development, 
before  it  has  had  time  to  soften,  will  effectually  do 
away  with  this  trouble,  or  indeed  a little  vigorous 
swishing  of  the  developer  back  and  forth  over  the 
plate  at  the  start  of  development  will  also  serve. 

74.  Black  Spots. — Occasionally  the  film  will 
have  small  black  marks  which  will  print  white. 
These  may  be  foreign  particles  sticking  to  the  film 
and  they  may  or  may  not  have  caused  an  extra 
deposit  of  silver  close  to  them.  It  is  well  to  move 
the  developer  over  the  film  a few  times  during  de- 
velopment to  aid  in  avoiding  such  marks,  but  the 
most  important  precaution  is  to  keep  all  trays  clean 
and  solutions  clear.  Always  pour  out  the  solutions 
gently  so  as  not  to  disturb  any  deposit  there  may 
be  at  the  bottom  of  the  bottle.  Care  in  keeping 
everything  clean,  trays,  desk,  shelf,  plate  holders, 
solutions,  will  banish  such  trouble.  If  the  plates 


NEGATIVE  DEFECTS  113 

frill  during  development  pieces  of  the  film  often 
become  detached  and  float  around  in  the  solution; 
in  which  case  it  is  well  to  keep  the  developer  mov- 
ing almost  continuously.  In  general  it  is  better  to 
rock  the  tray  only  occasionally.  In  rocking  the  tray 
do  not  lift  one  end  suddenly  and  then  drop  it  right 
away  again — watch  the  liquid  and  you  will  see  that 
it  has  not  moved.  But  raise  one  end  of  the  tray 
and  hold  it  up  for  a few  seconds,  then  lower  it  for 
a few  seconds,  and  the  liquid  will  have  time  to  flow 
over  the  plate.  Moving  the  liquid  helps  to  keep 
the  developer  alike  all  over  the  plate  and  so  helps 
to  secure  even  action. 

75.  Finger  marks  identify  themselves  readily, 
and  it  is  best  to  touch  the  surface  of  the  film  as 
little  as  possible,  particularly  if  the  skin  is  moist. 
When  necessary  to  tell  the  film  side  touch  only  the 
edges  or  corners. 

76.  Pressure  Marks. — If  the  sensitive  film  be 
subjected  to  pressure  at  any  time  it  will  deposit 
silver  at  these  places  during  development,  in  other 
words  it  will  develop.  The  plate  can  be  marked 
with  a hard  metal  point  just  before  placing  it  in 
the  developer,  and  the  marks  will  develop  along  with 
the  negative,  giving  a very  convenient  way  of  keep- 
ing track  of  plates.  For  this  reason  plates  are 
packed  so  that  the  films  are  held  apart  by  paper 
along  the  edges;  and  for  the  same  reason  it  is  wise 


PHOTOGRAPHY 


1 14 

to  stack  boxes  of  plates  on  edge.  The  same  thing 
is  often  troublesome  with  the  glossy  surfaced  de- 
veloping papers  but  much  less  often  with  the  matt 
surfaced.  The  deposits  in  the  case  of  the  glossy 
surfaced  papers  are  right  on  the  surface  and  may 
be  rubbed  off  with  some  absolute  alcohol  on  a clean 
cloth  without  injuring  the  print.  Also,  strangely 


enough,  the  presence  of  some  potassium  iodide  in 
the  developer  will  hold  back  the  development  of 
these  marks,  and  while  it  also  holds  back  the  regu- 
lar development,  it  does  not  do  so  as  much. 

77.  Halation. — The  spreading  of  the  light  from 
the  brightly  illuminated  parts  over  into  the  shad- 
ows is  called  halation.  It  will  hence  be  most  pro- 
nounced where  the  difference  in  illumination  across 
a narrow  line  is  great.  It  is  particularly  noticeable 


NEGATIVE  DEFECTS 


115 

around  windows  when  taking  interiors,  and  around 
the  fine  branches  of  trees  against  the  sky,  the  lake, 
or  snow.  The  fine  branches  will  not  appear  in  the 
print  unless  the  printing  exposure  is  made  too  great 
for  the  rest  of  the  print.  Also  in  pictures  showing 
the  sun  or  artificial  lights  the  effects  show  strongly. 
It  is  due  almost  entirely  to  two  causes,”^  (a)  internal 
reflections  in  the  glass,  and  (b)  irradiation.  Light 
reflected  from  the  outer  surface  of  the  film  strikes 
the  blackened  inside  of  the  camera  and  is  mostly 
absorbed.  In  the  passage  of  the  light  from  the 
film  to  the  glass  there  is  but  little  reflection,  on 
account  of  the  small  difference  in  densities,  and 
the  thinness  of  the  film  allows  but  little  spreading 
of  the  light  sideways.  But  when  the  light  strikes 
the  back  surface  of  the  glass,  the  next  medium  is 
air,,  so  that  there  is  a great  difference  in  density 
and  consequently  strong  reflection.  The  reflected 
light  strikes  the  back  of  the  sensitive  film  usually 
at  some  point  different  from  where  it  entered  and 
so  causes  a deposit  of  silver  where  there  should  not 
be  such  a one  in  the  picture  (Fig.  16).  It  will  be 
evident  immediately  that  it  will  show  most  along  the 
edge  of  a strongly  lighted  part  particularly  if  the 
adjoining  part  is  in  deep  shadow,  for  instance  in  the 
picture  of  a narrow  slit  with  a lamp  behind  it.  From 

^ Scheffer,  British  Jour,  of  Photo.,  57,  683  (1910)  ; Goldberg, 
Photographic  Journal,  51,  300  (1912). 


ii6 


PHOTOGRAPHY 


the  way  in  which  the  developer  has  to  enter  the  film, 
that  is  from  the  front  surface,  it  is  evident  that 
this  halation  exposure  will  not  have  a good  chance 
of  being  well  developed,  as  it  will  be  largely  at  the 
back  of  the  film;  so  that  when  you  suspect  halation 
trouble  do  not  develop  too  long,  which  will  subordi- 
nate the  halation  effect  by  lessening  the  contrasts 
generally  and  by  limiting  the  development  still  more 
of  the  halation  exposure.  When  possible  choose 
your  view  point  so  as  to  avoid  such  contrasts.  If 
they  are  unavoidable,  then  cover  the  back  of  the 
glass  with  something  which  will  absorb  the  light 
which  gets  through.  Black  paper  wet  with  glyc- 
erine and  rubbed  into  intimate  contact  with  the  glass 
will  do  well  if  the  plate  is  to  be  used  immediately. 
If  not  for  immediate  use  a paint  of  lampblack  and 
mucilage  is  better  but  is  a messy  job  to  apply;  plates 
can  be  bought  already  backed  thus.  Probably  the 
best  arrangement  of  all  is  to  put  the  absorbing  film 
in  between  the  gelatine  and  the  glass,  so  that  the 
light  is  absorbed  before  it  gets  to  the  glass.  This 
absorbing  film  can  be  made  of  a very  slow  emulsion 
(double-coated  plates)  or  of  a gelatine  film  carry- 
ing a brown  or  black  dye,  which  will  be  bleached  in 
the  acid  hypo,  leaving  the  plate  clear  for  printing.® 
See  Fig.  17,  page  144. 

® Such  for  example  as  the  “Non-halation  Simplex”  plates  of 
the  Lumiere  N.  A.  Co.,  Ltd.,  ii  W.  27th  St.,  New  York. 


NEGATIVE  DEFECTS 


117 


The  second  cause  of  the  spreading  of  the  light, 
irradiation,  is  the  scattered  reflection  from  the  par- 
ticles of  silver  bromide  in  the  emulsion,  as  in  the 
diagram,  thus  spreading  the  light  sideways  to  af- 
fect particles  which  should  not  be  affected  if  the 


Fig.  18. 


picture  is  to  be  true  geometrically  to  the  subject. 
This  of  course  will  show  most  distinctly  at  the  edges 
of  a brightly  lighted  area,  for  while  it  is  present  all 
over  the  area  it  will  be  uniform  and  not  so  easily 
observed.  There  is  no  remedy  for  it.  Fortunately 
it  is  not  a very  serious  matter  in  its  effect  on  the 
picture.  The  glass  reflection  part  is  a good  deal 
more  serious.  Halation  shows  very  little  in  nega- 


ii8 


PHOTOGRAPHY 


tives  where  the  sensitive  film  is  carried  on  the  thin 
celluloid  sheet,  as  in  roll-films  and  film-packs. 

78.  ‘‘Oyster  Shell  Markings.”— In  the  laboratory 
a number  of  times  now  we  have  run  across  very 
peculiar  markings  on  some  plates  and  paper.  It 
consists  of  wavy  somewhat  parallel  groups  of  lines 
often  spread  over  the  whole  plate.  They  resemble 
flow  lines  in  a viscous  fluid  or  the  ‘^oyster  shelF’ 
markings  in  collodion  plates.  So  far  I have  not 
been  able  to  find  any  satisfactory  explanation  of 
their  cause  or  cure. 

79.  Stain. — With  most  organic  developers,  and 
especially  with  pyro,  the  gelatine  film  is  stained 
somewhat  during  development.  With  prolonged 
development  in  pyro  it  may  be  quite  pronounced. 
Sodium  sulphite  in  the  developer  helps  to  check  it, 
and  the  acid  along  with  sodium  sulphite  in  the 
hypo  bath  removes  a large  part  but  not  all.  It  is 
of  no  particular  harm  if  not  so  dense  as  to  unduly 
prolong  the  time  of  printing. 

80.  Bubbles. — Mention  has  already  been  made 
of  the  formation  of  small  bubbles  right  in  the  gela- 
tine film  if  the  hypo  bath  is  too  warm,  particularly 
if  the  bath  is  old.  The  bubbles  do  not  disappear 
on  drying,  and  if  in  a negative  will  show  in  the  print 
made  from  it.  The  same  sort  of  thing  shows  occa- 
sionally in  prints,  particularly  in  bromide  enlarge- 
ments, caused  apparently  by  too  great  changes  in 


NEGATIVE  DEFECTS 


119 

temperature  in  the  different  baths,  or  by  too  strong 
hypo  bath. 

81.  Drying  Troubles.— In  addition  to  the  care 
necessary  to  remove  the  deposit  which  settles  from 
the  wash  water  on  the  film  before  putting  it  up  to 
dry,  there  are  several  other  things  to  attend  to  in 
the  drying  of  the  film.  Prevent  the  settling  of  dust 
on  the  wet  surface  of  the  gelatine  by  setting  it  on 
edge  to  dry,  or  even  set  sloping  film  side  down. 
The  film  sags  slightly  during  drying  so  that  it  is 
best  if  it  dries  at  a uniform  rate  and  in  one  position, 
which  will  give  the  least  distortion  of  the  picture 
and  least  chance  for  variations  in  thickness.  The 
film  may  be  dried  somewhat  quickly  by  leaning  it 
film  side  toward  a cold  window  with  a warm  room 
behind.  The  water  will  distill  rapidly  from  the 
film  and  condense  on  the  window.  Of  course,  if 
one  tries  to  hasten  the  drying  by  warming,  the  film 
will  melt  and  the  negative  be  ruined.  If  the  wet 
plate  be  immersed  in  alcohol  containing  very  little 
water,  the  water  in  the  film  will  pass  out  quickly 
and  almost  completely  into  the  alcohol,  the  film  los- 
ing its  gummy  feel  and  becoming  tough  and  hard. 
When  the  alcohol  is  dried  off  the  film  will  stand 
gentle  heating  to  finish  the  drying,  but  is  still  quite 
easily  melted.  If  the  wet  film  be  immersed  in  a 
solution  of  formaldehyde  it  will  harden  the  gelatine 


120 


PHOTOGRAPHY 


and  render  it  insoluble  in  water  so  that  the  film  may 
be  dried  by  quite  vigorous  heating. 

For  further  reading  an  excellent  list  and  discus- 
sion of  plate  troubles  is  given  by  Olaf  Bloch  in  the 
Photographic  Journal,  55,  219  (1915), 


CHAPTER  VI 


POSITIVE  PROCESSES 

In  connection  with  the  work  in  the  laboratory  it 
will  be  advisable  to  describe  a few  of  the  very  great 
many  methods  of  making  positives. 

82.  Transparencies. — The  most  obvious  way  to 
make  a positive  is  by  exposing  another  plate  either 
through  a lens  or  by  contact,  to  the  light  transmit- 
ted by  a negative.  The  larger  sizes  made  this  way 
go  by  the  name  of  transparencies,  and  when  the 
glass  support  of  the  film  is  translucent  milk  glass, 
the  effect  is  in  some  respects  far  superior  to  prints 
on  paper.  The  total  scale,  that  is  the  difference  be- 
tween the  greatest  blackness  and  the  greatest 
brightness,  is  many  times  greater  for  the  plate, 
which  gives  them  a luminosity  and  detail  in  the 
shadows  unapproachable  in  a print.  On  the  other 
hand  they  are  more  clumsy  to  view  and  more  bulky 
to  store. 

83.  Lantern  Slides. — -The  small  size  used  in  the 
lantern  are  the  most  important  transparencies. 
American  slides  are  made  3^  inches  by  4 inches, 
while  English  ones  are  3^  by  3j4>  French  are  8 

121 


122 


PHOTOGRAPHY 


by  lo  centimeters — 3.54  by  4.72  inches — and  gen- 
eral purpose  lanterns  should  be  provided  with  slide 
carriers  to  take  all  these  sizes. 

All  lanterns  are  made  to  take  the  slides  long  way 
horizontal  and  it  is  hence  necessary  to  make  the 
horizontal  lines  of  the  subject  run  the  long  way  of 
the  slide.  For  a good  slide  the  grain  of  the  reduced 
silver  should  be  very  fine,  which  requires  a fine- 
grained emulsion,  and  these  are  slow  ones.  In  con- 
sequence lantern  slide  plates*  are  usually  especially 
made  slow  plates  of  fine  grain,  having  a thin  layer 
of  emulsion  on  a good  grade  of  glass.  The  thinness 
of  the  layer  makes  them  cheaper  and  is  better  since 
such  dense  deposits  are  not  required,  as  in  negatives, 
nor  is  such  latitude  in  the  exposure  necessary. 
Lantern  slides  are  better  when  they  have  consider- 
able contrast,  and  therefore  the  negatives  from 
which  lantern  slides  are  to  be  made  can  usually 
be  developed  for  longer  than  usual  with  advantage, 
particularly  where  the  subject  is  black  and  white, 
as  in  line  drawings  or  ordinary  printing. 

It  must  be  observed  that  the  general  properties  of 
the  gelatine  plate,  as  already  described  in  some  de- 
tail, are  no  different  in  the  case  of  lantern  slide 
plates.  Contrast  is  determined  by  the  subject  and 
the  amount  of  development,  so  that  the  maximum 
density  must  be  controlled  by  the  exposure.  The 
maximum,  density  is  always  much  less  than  the 


POSITIVE  PROCESSES 


123 


thinnest  usable  negative.  The  one  distinctly  new 
demand  is  that  the  clear  places  must  be  free  from 
deposit,  and  hence  these  places  must  receive  an 
exposure  of  about  half  the  inertia.  Practically 
in  the  laboratory  this  means  the  least  exposure, 
which,  on  a generous  amount  of  development,  will 
give  sufficient  maximum  density.  Too  long  an  ex- 
posure can  not  be  corrected  by  quick  removal  from 
the  developer,  but  this  will  always  result  in  a gen- 
eral deposit  and  in  lack  of  contrast.  If  the  nega- 
tive has  sufficient  contrast  and  the  development  has 
not  been  excessive,  a deposit  in  the  clear  places 
means  too  long  an  exposure.  To  avoid  a deposit 
in  the  clear  places  a developer  giving  very  little  fog 
is  desirable,  and  on  this  account  the  ferrous  oxalate 
developer  ^ is  without  equal.  It  has  the  disadvantage 
that  it  spoils  quickly  when  exposed  to  the  air  and 
it  is  best  mixed  fresh  for  each  plate.  It  is  also 
necessary  to  wash  out  the  developer  or  rather  the 
iron  salts  in  it,  by  following  a rinse  under  the  tap 
with  a soaking  for  a few  minutes  in  a solution  of 
oxalic  acid,  as  otherwise  the  clear  places  may  show 
a deposit  of  an  iron  compound  which  will  not  wash 
out  in  water.  The  acid  in  the  fixing  bath  also  helps 
to  remove  any  such  deposit.  Most  other  lantern 
slide  developers  are  loaded  with  bromide,  which 


^ See  Manual,  Exp.  7. 


124 


PHOTOGRAPHY 


while  suppressing  fog  also  suppresses  the  lower 
densities,  and  so  interferes  with  the  gradation.  But 
with  the  ferrous  oxalate  the  advantage  of  clear 
places  is  accompanied  by  beautiful  full  scale  grada- 
tion so  that  the  extra  trouble  if  any  is  well  repaid. 

Lantern  slides  made  by  the  wet  collodion  proc- 
ess are  still  ranked  as  perhaps  the  best,  as  they  have 
very  clear  high  lights  and  abundant  detail.  They 
require  however  a great  deal  of  care  and  skill  for 
first  class  work,  and  are  not  often  seen  today. 

84.  Blue  Prints. — Probably  the  simplest  of  all 
the  photographic  processes  for  making  prints  on 
paper  is  the  ''blue  print.’’  In  it  the  light-sensitive 
substance  is  a salt  of  iron  in  the  ferric  condition. 
Ferric  ammonium  citrate  is  the  one  frequently  used, 
and  the  action  of  light  on  it  is  to  change  the  iron 
over  to  the  ferrous  form.  Both  ferric  and  ferrous 
salts  are  usually  only  slightly  colored  so  that  the 
image  has  to  be  "developed,”  that  is  the  compound 
formed  by  the  light  has  to  be  changed  into  an  in- 
soluble colored  compound.  The  substance  gener- 
ally used  for  this  purpose  is  potassium  ferricyanide, 
which  forms  a deep  blue,  insoluble  compound  called 
Turnbull’s  Blue  with  the  ferrous  salt,  while  it  forms 
no  insoluble  compound  with  the  ferric  salt.  The 
paper  having  a suitable  surface  to  carry  the  colored 
image  is  brushed  over  with  a solution  of  the  two 
salts  dissolved  in  water  and  dried  quickly  and  thor- 


POSITIVE  PROCESSES 


I2S 


oughly.^  Exposed  to  sunlight  under  a negative,  it 
turns  blue  showing  a visible  image;  the  depth  of 
printing  has  to  be  found  by  trial.  If  the  paper  be 
now  dipped  in  water  the  ferrous  salt  formed  by  the 
action  of  the  light  will  react  with  the  potassium  fer- 
ricyanide,  where  it  has  not  already  done  so,  chang- 
ing the  ferrous  image  into  Turnbulhs  Blue,  and  the 
extra  unused  materials  will  be  washed  out,  thus 
'"fixing’^  the  image.  The  paper  does  not  keep  very 
well  after  it  is  coated,  and  when  it  begins  to  go 
bad  the  whites  will  be  tinged  with  blue.  Keeping 
the  paper  thoroughly  dry  retards  the  deterioration 
very  much,  and,  conversely,  dampness  ruins  it  quick- 
ly. It  is  advisable  but  not  at  all  necessary  that  the 
coating  should  be  uniform,  as  quite  streaky  papers 
will  give  good  prints  if  there  is  sufficient  supply  of 
the  salts  at  all  places,  so  that  the  light  will  have 
plenty  to  act  upon,  and  the  excess  disappears  in  the 
washing.  Not  all  papers  are  suitable  for  coating; 
a tough  paper  with  a good  surface  is  required,  and 
often  it  will  pay  to  size  it  by  coating  with  albumen 
or  gelatine. 

85.  Printing  Out  Paper  (P.  O.  P.). — It  is  also 
called  gelatino-chloride  paper.  This  is  made  by 
coating  paper  with  a layer  of  gelatine  in  which  is 
present  some  soluble  chloride  like  sodium  chloride. 
When  dry  this  is  floated  on  a solution  of  silver  ni- 
* See  Manual,  Exp.  16. 


126 


PHOTOGRAPHY 


trate,  when  the  silver  salt  reacts  with  the  chloride  to 
give  the  insoluble  silver  chloride,  and  the  other 
compounds  present  at  first  or  formed  are  removed 
by  washing.  The  gelatine  may  be  partially  or  en- 
tirely replaced  by  albumen,  giving  the  albumeno- 
chloride  papers.  The  treatment  in  all  cases  is  very 
much  alike.^  They  all  require  sunlight  for  print- 
ing and  give  a red  image  which  fades  out  materi- 
ally in  the  fixing;  if  the  hypo  is  used  too  strong  the 
bleaching  is  serious.  The  red  color  is  usually  not 
considered  a pleasing  tone,  and  there  is  some  doubt 
as  to  the  permanence  of  the  red  image,  but  if  it  is 
made  with  care,  especially  by  the  use  of  fresh  hypo, 
and  thorough  washing,  it  is  probably  permanent. 
To  change  the  color  and  to  add  permanence  it  is 
customary  to  exchange  the  silver  forming  the  image 
for  gold  or  less  frequently  for  platinum.  Simple 
contact  between  the  silver  in  the  image  and  the 
gold  or  platinum  salt  in  solution  is  sufficient  to 
cause  them  to  exchange  places,  the  silver  going  into 
solution  and  the  gold  changing  from  the  compound 
to  metallic  gold,  or  metallic  platinum  in  case  a 
platinum  salt  is  used.  The  image  in  either  case  is 
very  permanent.  With  gold  the  color  or  tone  may 
be  varied  through  red  to  purple  to  a good  black. 
Most  photographers’  proofs  used  to  be  made  on  this 
kind  of  paper,  but  of  a very  poor  grade  and  not  very 
® See  Manual,  Exp.  17. 


POSITIVE  PROCESSES  127 

suitable  for  toning.  ''Solio’’  is  one  of  the  best 
known  makes  of  this  paper. 

In  the  past  photo-engravers  much  preferred 
prints  on  this  paper  from  which  to  make  cuts.  They 
liked  the  color,  the  glossy  surface,  and  the  wealth  of 
detail.  Opinion  is  changing  somewhat,  and  prints 
on  glossy  developing  paper,  where  the  image  has  a 
green  tone,  are  now  often  asked  for.  Photogra- 
phers generally  still  rank  prints  on  printing  out 
paper  ahead  of  prints  on  developing  paper,  but  the 
former  requires  more  time  and  labor,  so  that  they 
have  gone  largely  out  of  general  use. 

86.  Developing  Paper. — At  the  present  time  this 
is  by  all  means  the  most  popular  printing  method 
both  among  amateurs  and  professionals.  Its  popu- 
larity is  justified  by  its  ease  and  certainty  of  work- 
ing, by  the  permanence  of  the  prints,  and  by  the 
great  variety  of  surface  and  of  ability  to  render 
contrast  which  may  be  obtained.  The  paper  is 
coated  with  an  emulsion  which  is  very  similar  to 
the  plate  emulsions  except  that  it  is  much  finer 
grained  and  slower.  Besides  this  it  usually  has  pre- 
servatives added  to  improve  its  keeping  qualities, 
and  good  prints  can  often  be  made  on  paper  years 
old.  Two  distinct  grades  are  on  the  market,  one 
called  ''bromide  paper,’'  designed  for  enlarging 
work  and  therefore  having  a fast  emulsion,  the 
other  slower,  designed  for  contact  printing.  By 


128 


PHOTOGRAPHY 


the  choice  of  the  paper  on  which  either  is  coated 
and  by  manipulation  of  the  emulsions,  the  surface 
of  the  dry  print  may  be  varied  from  a very  brilliant 
gloss  gradually  to  a very  coarse  pebble  or  matt. 
The  choice  of  the  different  surfaces  is  determined 
by  the  taste  of  the  worker  and  the  character  of  the 
picture;  where  great  detail  is  desired  the  glossy 
surfaces  are  best,  and  where  less  detail  is  desired, 
as  in  portraits,  some  one  of  the  matt  surfaces  is  to 
be  preferred.  It  is  worthy  of  note  also  that  the 
amount  of  contrast  which  the  paper  can  render  is 
greater  in  the  case  of  the  glossy  surface.  Besides 
this  range  of  surfaces,  most  makers  turn  out  sev- 
eral different  grades  of  paper,  which  differ  in  their 
ability  to  render  contrast,  some  designed  to  give 
strong  contrast  for  use  with  flat  negatives  or  where 
harsh  prints  are  desired,  the  others  grading  over  to 
a paper  intended  to  lessen  the  contrasts  in  too  harsh 
negatives.  Skillful  use  of  these  varieties  gives  the 
worker  great  command  over  the  character  of  the 
print. 

Ever  since  Hurter  and  Driffield’s  work  with  plates 
efforts  have  been  made  to  systematize  the  prop- 
erties of  developing  paper.  This  has  finally  been 
accomplished  ^ and  the  results  are  of  great  interest 
to  the  user  of  the  paper  as  well  as  to  the  manufac- 
turer. Since  prints  are  observed  by  reflected  light 
Hones,  Nutting  & Mees,  British  Journal^  62,  9 (1915). 


POSITIVE  PROCESSES 


129 


while  negatives  are  used  always  with  transmitted 
light,  the  ''density’'  of  the  image  will  have  a some- 
what different  meaning  in  the  two  cases.  The 
curves  are  very  similar  to  the  plate  curves  and  the 
conclusions  with  regard  to  exposure  and  develop- 
ment are  also  very  similar.  As  a rule  the  papers 
develop  very  rapidly,  so  that  the  value  of  gamma 
quickly  reaches  a maximum;  with  the  usual  bro- 
mided  developers  the  further  progress  of  develop- 
ment simply  shifts  the  curve  parallel  to  itself.  The 
contrast  of  the  paper  is  fixed  by  the  slope  of  this 
line  and  its  projection  on  the  exposure  axis.  Usu- 
ally as  this  slope  increases  with  different  papers  the 
latitude  of  the  paper  decreases.  As  with  plates  a 
portion  of  the  bent  ends  of  the  characteristic  curve 
is  used  in  practice,  and  the  projection  of  these 
straight  and  curved  parts  on  the  exposure  axis  is 
called  the  ''total  scale/'  As  would  be  expected  the 
total  scale  of  hard  papers  is  much  smaller  than  that 
of  soft  papers.  On  account  of  the  rapidity  of  devel- 
opment it  is  not  possible  to  control  the  contrast 
materially  by  varying  the  development.  The  only 
control  is  by  choice  of  the  grade  of  paper. 

In  most  cases  the  printing  method  is  at  its  best 
only  with  a particular  amount  of  contrast  in  the 
negative.  In  fact  such  an  amount  that  the  differ- 
ence in  the  printing  exposures  between  the  densest 
and  the  thinnest  parts  of  the  negative  should  be 


130 


PHOTOGRAPHY 


such  that  when  the  thinnest  part  exposes  the  paper 
for  its  maximum  density,  the  exposure  under  the 
dense  part  will  be  just  not  sufficient  to  show  gray- 
ness. That  is  the  amount  of  contrast  in  the  nega- 
tive should  be  such  as  to  give  a series  of  exposures 
to  the  print  as  nearly  equal  as  possible  to  the  full 
scale  of  the  paper.  Otherwise  part  of  the  detail  in 
the  negative  will  be  lost — a very  common  case — 
or  use  will  not  be  made  of  the  full  capabilities  of 
the  paper.  The  latitude  of  a plate  always  exceeds 
the  variation  in  the  light  intensities  in  the  ordinary 
subject  so  that  the  plate  can  render  truthfully  the 
full  scale  in  the  subject.  But  with  printing  papers 
this  is  usually  not  so,  the  total  scale  of  the  paper 
being  too  short.  In  such  cases  one  can  reduce  the 
gamma  of  the  negative  to  less  than  unity  by  lessen- 
ing its  development,  or  sacrifice  detail  at  one  or 
other  end  of  the  scale.  To  illustrate,  by  lengthen- 
ing the  printing  exposure  with  an  ordinary  nega- 
tive the  clouds  may  usually  be  shown  on  an  ordi- 
nary print,  but  then  the  whole  foreground  will  be 
black;  while  if  the  printing  exposure  be  short 
enough  to  show  the  detail  in  the  foreground,  then 
the  sky  will  print  white.  But  if  the  negative  had 
been  developed  for  a shorter  time,  thus  lessening  the 
amount  of  contrast,  it  may  be  reduced  to  the  total 
scale  of  the  paper,  so  that  the  sky  and  foreground 
detail  will  both  appear  in  the  same  print.  It  may 


POSITIVE  PROCESSES 


131 

also  be  possible  to  select  a paper  with  a greater 
total  scale,  and  so  accomplish  the  same  end. 

A developer  such  as  the  ferrous  oxalate,  or  metol, 
* or  hydrochinon,  etc.,  which  will  not  stain  the 
paper  is  required.  An  acid  hypo  will  aid  in  avoid- 
ing any  stain  from  the  developer,  and  the  harden- 
ing agent,  alum,  will  aid  in  avoiding  trouble  in  any 
subsequent  treatment  of  the  print.  Care  is  required 
in  the  use  of  the  hypo  bath,  that  it  be  kept  clean 
and  reasonably  fresh,  and  that  none  of  it  be  allowed 
to  get  into  the  developer.  Yellow  stains  in  the 
print  will  be  traced  most  often  to  the  presence  of  a 
small  quantity  of  hypo  in  the  developer;  these  stains 
often  do  not  show  or  are  not  noticed  till  the  batch 
of  prints  is  dry,  so  that  the  trouble  escapes  notice 
at  the  time  when  it  could  be  remedied  by  the  use  of 
fresh  developer.  It  is  one  of  the  cases  where  care 
and  cleanliness  bring  an  ample  reward.  Refer  to 
the  Manual,  Exp.  4,  for  further  laboratory  details. 

The  color  of  the  image  is  ordinarily  a shade  of 
gray  or  black.  If  the  developer  contains  a large 
proportion  of  soluble  bromide,  say  potassium  bro- 
mide, the  image  will  have  a marked  green  shade. 
If  the  ordinary  metol-hydrochinon  developer  is 
made  with  little  or  no  carbonate,  the  image  will  be 
a shade  of  brown  with  lessened  contrasts.  Besides 
these  shades  the  color  of  the  image  in  the  finished 
picture  may  be  changed  to  one  of  many  other  colors 


132 


PHOTOGRAPHY 


by  the  use  of  suitable  reagents,  and  of  these  meth- 
ods we  will  consider  only  one. 

87.  This  process  called  ''Sulphide  toning''^ 
changes  the  silver  in  the  image  into  silver  sulphide, 
which  has  a brown  color  when  divided,  as  it  is  in 
the  print.  It  is  not  feasible  to  change  the  metallic 
silver  directly  into  the  sulphide,  so  that  the  process 
has  to  be  carried  on  in  two  steps,  first  the  change 
of  the  metallic  silver  into  some  insoluble  silver  salt, 
usually  the  bromide,  and  second  the  change  of  this 
salt  into  the  sulphide.  The  well-washed  print  is 
first  immersed  in  a solution  of  potassium  ferricya- 
nide  and  potassium  bromide.  The  ferricyanide  at- 
tacks the  silver  forming  the  potassium  silver  ferro- 
cyanide,  thus, 

Ag2+2K3Fe(CN)6  = K4Fe(CN)6+K2Ag2Fe(CN)6 

and  the  potassium  bromide  changes  this  over  to 
silver  bromide  on  account  of  the  much  greater  in- 
solubility of  the  latter,  thus: 

K2Ag2Fe(CN)e+ 2KBr  = K4Fe(CN)s+ 2AgBr 

The  silver  bromide  is  a pale  yellow  so  that  the  result 
of  this  is  to  change  the  black  silver  image  to  an 
almost  invisible  one.  The  bleached  print  has  then 
to  be  thoroughly  washed  to  free  it  from  all  of  these 
substances  except  the  silver  bromide,  which  is  too 
® Manual,  Exp.  19. 


POSITIVE  PROCESSES 


133 


insoluble  to  wash  out.  The  second  step  in  the  proc- 
ess is  to  immerse  the  washed  bleached  print  in  a 
solution  of  some  soluble  sulphide,  such  as  sodium 
sulphide,  Na2S,  when  the  silver  bromide  will  be 
changed  into  the  brown  silver  sulphide,  thus : 

Na2S-l-  2AgBr  = Ag2S + 2NaBr 

this  change  also  being  induced  by  the  greater  in- 
solubility of  silver  sulphide  than  of  silver  bromide. 
Shades  between  brown  and  black  may  be  obtained 
by  partially  developing  the  bleached  image  in  any 
paper  developer,  and  then  changing  the  remaining 
silver  bromide  into  silver  sulphide,  as  above. 

88.  Platinotype.  — This  is  a process  which  is 
closely  allied  to  the  Blue  Print  Process,  and  is  often 
referred  to  as  platinum  paper.  In  this  connection 
it  is  well  to  note  that  a number  of  the  commercial 
papers  described  as  platinum  papers  are  only  imi- 
tations of  them,  the  paper  being  really  a slightly 
modified  developing  paper.  Platinum  is  an  exceed- 
ingly expensive  metal,  three  or  four  times  the  cost 
of  gold,  and  it  takes  enough  of  it  to  make  a satis- 
factory paper  that  it  is  a very  important  item,  so 
that  there  is  a great  incentive  to  substitute  some 
less  expensive  material.  The  modified  developing 
paper  is  a very  good  imitation.  The  light  sensitive 
substance  in  real  platinum  paper  is  a ferric  salt, 
usually  ferric  oxalate,  Fe2 (€204)3,  which  by  the 


134 


PHOTOGRAPHY 


action  of  the  light  changes  over  to  ferrous  oxalate. 
The  ferric  salt  is  mixed  with  a salt  of  platinum, 
usually  platinic  chloride  PtCh,  and  the  solution 
coated  on  paper  exactly  as  with  blue  print  paper. 
The  surface  of  the  paper  is  often  not  suitable,  in 
which  case  it  has  to  be  coated  with  something  which 
will  hold  the  particles  of  platinum  forming  the  fin- 
ished image.  The  coating  may  be  done  with  vari- 
ous substances,  as  starch,  or  gelatine,  by  floating 
the  paper  on  the  solution  of  one  of  these  and  then 
drying  it.  After  sensitizing  the  paper  has  to  be 
dried  as  quickly  as  possible,  and  to  have  it  keep 
good  for  even  a few  days  it  must  be  kept  exceed- 
ingly dry. 

The  commercial  makers  have  some  tricks  for  aid- 
ing the  keeping  qualities,  but  they  all  have  to  be 
kept  dry  also.  For  this  purpose  the  paper  is  usually 
kept  in  closed  boxes,  where  there  is  also  placed  some 
water-absorbing  agent,  such  as  calcium  chloride. 
In  spite  of  all  care  the  paper  will  only  keep  a few 
weeks,  after  which  it  will  have  a deposit  in  the  clear 
parts  of  the  image,  that  is,  show  fog.  It  has  to  be 
printed  in  direct  sunlight,  and  the  image  shows  only 
slightly,  so  that  the  progress  of  printing  has  to  be 
followed  by  some  kind  of  exposure  meter,  such  for 
example,  as  a piece  of  solio  under  another  negative 
but  exposed  to  the  same  sunlight.  It  is  much  better 
to  use  a constant  light  source,  as  the  arc  or  a high- 


POSITIVE  PROCESSES 


135 


power  tungsten  lamp,  when  the  time  of  exposure 
gives  a direct  measure  of  the  progress  of  printing. 
After  exposure  the  paper  is  developed  by  floating 
face  downward  on  a solution  of  potassium  oxalate 
in  which  the  iron  salts  are  soluble.  In  solution  fer- 
rous salts  quickly  reduce  the  platinum  salt  to  metal- 
lic platinum  with  the  simultaneous  formation  of 
ferric  salt  again.  This  reaction  takes  place  only 
where  the  ferrous  salt  has  been  formed,  so  that  the 
ferrous  image  is  changed  into  a metallic  platinum 
image.  The  print  must  be  washed  carefully  to  rid 
it  of  all  iron  salts.  Such  prints  are  very  permanent, 
but  they  can  hardly  be  more  permanent  than  the 
paper  support,  and  a*  good  developing  paper  is  as 
permanent  as  that.  This  paper  lends  itself  readily 
to  a lot  of  different  surfaces  and  grains,  and  par- 
ticularly readily  to  the  production  of  pictures  in 
which  the  fine  details  are  run  together  to  give  large 
light  and  shade  effects — what  are  often  referred  to 
as  ''artistic”  prints.  In  special  cases  there  are  ad- 
vantages to  be  gained  by  this  sacrifice  of  detail,  but 
in  a great  many  of  the  pictures  one  sees  the  gain 
is  very  doubtful.® 

89.  Carbon  Printing. — Tn  this  process  we  come 
back  to  the  use  of  gelatine,  but  this  time  it  is  the 
light  sensitive  material  itself.  It  is  based  upon  the 


“ For  a more  extended  discussion  of  Platinotype  see  Phot. 
Miniature  No.  7. 


136  PHOTOGRAPHY 

property  that  if  it  be  bathed  in  a solution  of  a salt 
of  chromic  acid,  like  potassium  chromate  or  ammo- 
nium chromate,  and  then  dried,  the  gelatine  remains 
soluble  in  warm  water,  but  if  while  still  dry  it  be 
exposed  to  light,  the  gelatine  becomes  insoluble  in 
warm  water.  In  consequence  a film  of  gelatine  sen- 
sitized with  the  chromate  may  be  exposed  to  sun- 
light under  a negative,  and  the  parts  left  soluble 
can  be  washed  away  with  warm  water,  leaving  a 
positive.  The  gelatine  film  may  be  colored  with 
dyes  or  carbon  (lamp  black)  so  that  the  resulting 
print  can  be  any  tone  desired  and  the  same  tone 
may  be  duplicated  any  number  of  times  whenever 
desired.  It  will  be  evident  from  the  way  the  ex- 
posure is  made  that  the  insoluble  image  will  be  on 
the  surface  of  the  gelatine  film  and  extending  down 
into  the  unaffected  gelatine  and  probably  in  no 
places  extending  right  through  to  the  paper  sup- 
port. So  that  when  treated  with  warm  water  the 
whole  image  will  float  off.  To  avoid  this  it  is  nec- 
essary to  support  the  image  on  the  exposed  side  by 
transferring  it  to  a support  in  contact  with  this 
surface.  This  is  very  readily  accomplished  by 
bringing  the  wet  image  surface  in  contact  with  a 
wet,  hardened,  gelatine  surface,  and  when  the  excess 
of  water  is  pressed  out  they  will  adhere  quite  suffi- 
ciently to  preserve  the  image  while  the  original  sup- 
port is  soaked  off  with  warm  water  and  then  the 


POSITIVE  PROCESSES 


137 


remaining  soluble  gelatine  washed  away.  This 
transfer  reverses  the  image,  making  it  right  left- 
handed,  and  to  bring  it  back  to  agreement  with  the 
subject  it  requires  another  transfer.  In  some  cases, 
as  portraits  for  example,  this  reversal  does  no  harm 
but  in  case  it  is  necessary  it  is  readily  done.  If 
the  surface  to  which  the  print  is  first  transferred 
be  previously  rubbed  over  with  a small  quantity  of 
beeswax  dissolved  in  turpentine,  the  developed 
print  after  drying  will  leave  it  usually  spontane- 
ously; the  final  support,  usually  a soft  gelatine  sur- 
face, has  to  be  rubbed  into  contact  with  the  devel- 
oped print  while  all  are  still  wet,  and  all  allowed  to 
dry  in  contact;  when  dry  the  temporary  support 
splits  off  easily. 

The  tissue  as  supplied  by  the  dealers  is  simply  a 
thin  layer  of  colored  gelatine  spread  over  a strong 
paper,  and  in  this  condition  it  will  usually  keep  for 
years.  When  wanted  it  has  to  be  sensitized  by  bath- 
ing in  the  solution  of  the  chromic  salt,  or  by 
brushing  the  solution  over  the  surface  of  the  gela- 
tine, or  the  most  convenient  way  is  to  brush  over 
the  surface  an  alcoholic  solution  of  ammonium 
chromate,  which  dries  off  quickly,  and  the  tissue  is 

^ Autotype  supplies  for  all  kinds  of  carbon  printing  may  be 
obtained  from  George  Murphy,  Inc.,  57  East  9th  St.,  New 
York,  who  also  can  supply  several  good  books.  Tissue  on  a 
transparent  celluloid  support  may  be  obtained  from  the  Neue 
Photographische  Gesellschaft,  Berlin,  Steglitz. 


PHOTOGRAPHY 


138 

ready  for  exposing  in  an  hour  or  two.  The  film 
must  be  thoroughly  dried  and  then  kept  as  dry  as 
possible  or  it  will  fog  in  a few  days  or  even  hours. 
It  is  best  kept  in  a closed  box  in  the  presence  of 
calcium  chloride.  Even  with  all  precautions  it  only 
keeps  in  good  condition  for  a week  or  two,  so  that 
it  has  to  be  sensitized  when  needed. 

As  with  platinotype  paper,  the  progress  of  print- 
ing cannot  be  seen,  as  the  image  is  almost  invisible, 
and  it  has  therefore  to  be  followed  with  some  kind 
of  an  exposure  meter  when  the  source  of  light  is  a 
variable  one  like  sunlight.  If  a steady  source,  like 
a good  arc  lamp,  be  used  a few  trials  will  determine 
the  time  required.  The  tissue  can  also  be  bought 
where  the  gelatine  layer  is  carried  on  a thin,  trans- 
parent celluloid  support  and  the  printing  can  be 
done  through  the  support,  thus  saving  the  trouble 
of  the  first  transfer. 

The  process  requires  more  labor  and  skill  than 
the  ordinary  developing  paper  process,  but  is  not 
difficult  enough  to  be  beyond  the  reach  of  any  ordi- 
nary worker.  The  method  is  exceedingly  flexible 
as  to  the  rendering  of  fine  detail  or  large  fuzzy 
effects  depending  on  the  treatment  in  the  warm 
water,  and  as  to  the  tone  of  the  finished  print.  It 
can  be  used  to  advantage  in  lantern-slide-making 
for  the  same  reasons,  and  has  a very  important  ap- 


POSITIVE  PROCESSES 


139 

plication  in  the  making  of  pictures  in  their  natural 
colors. 

90.  Photo-engraving. — There  are  many  printing 
press  methods  which  come  naturally  under  this 
head.  But  the  great  majority  of  newspaper,  mag- 
azine, and  book  illustrations  are  made  by  methods 
of  which  the  half-tone  zinc  etching  may  be  taken  as 
a type.  The  first  step  is  to  make  a negative  of  the 
subject  to  be  copied.  In  this,  one  detail  is  different 
from  the  ordinary  process.  At  a short  but  definite 
distance  in  front  of  the  plate  in  the  camera  is  placed 
a line  screen  made  of  two  sets  of  parallel  lines,  the 
sets  being  at  right  angles  to  each  other,  and  in  each 
set  the  width  of  the  opaque  lines  and  the  clear  places 
are  about  equal.  This  breaks  up  the  light  image 
into  a series  of  dots,  but  on  account  of  the  separa- 
tion of  the  line  screen  and  plate,  the  dot  on  the 
plate  is  brightest  at  the  center  and  falls  off  gradu- 
ally instead  of  ending  sharply.  The  result  of  this 
in  the  negative  is  to  make  the  dots  large  in  the 
brightly  illuminated  parts  and  small  in  the  dark 
places.  For  the  best  effect  these  dots  have  to  be 
very  opaque  and  clearly  marked  so  that  the  nega- 
tive has  usually  been  made  by  the  wet  collodion 
process,  which  gives  very  clear  shadows  and  allows 
readily  of  very  great  intensification. 

A print  is  made  from  this  negative  as  in  the 
carbon  process,  only  using  a very  thin  layer  of  bi- 


140 


PHOTOGRAPHY 


chromated  fish-glue  supported  on  a polished  zinc 
plate,  and  the  exposure  is  made  long  enough  that  the 
insolubility  extends  right  through  to  the  metal. 
This  is  developed  in  warm  water,  dried,  and  the 
plate  heated  to  almost  char  the  fish-glue,  which 
makes  it  very  hard  and  also  insoluble  in  the  acid 
bath  into  which  the  plate  is  next  dipped.  Where 
exposed  the  zinc  is  eaten  away  and  special  pre- 
cautions have  to  be  taken  to  prevent  the  acid  eat- 
ing under  the  edges  of  the  glue.  The  hardened 
glue  is  left  on  as  it  forms  a good  wearing  surface, 
the  plate  dried,  inked  by  passing  an  inky  roller  over 
it,  and  the  inked  plate  pressed  against  paper.  The 
unetched  parts  of  the  surface  will  transfer  ink  to 
the  paper,  while  the  etched  parts  will  not  touch  it. 
The  unetched  parts  correspond  to  insoluble  glue, 
these  to  clear  places  in  the  negative,  these  to  dark 
places  in  the  original  subject,  so  that  dark  places  in 
the  original  subject  are  represented  by  inky  places 
in  the  print.  Where  the  light  is  strong  in  the 
image,  the  silver  dots  in  the  negative  spread  out 
and  overlap,  leaving,  therefore,  clear  dots  on  a black 
ground;  and  where  the  light  in  the  image  is  weak, 
the  silver  dots  in  the  negative  stay  small,  appearing, 
therefore,  as  black  dots  on  a clear  ground.  In  the 
positive  image  in  glue  and  in  ink,  the  clear  and  black 
places  have  been  reversed,  but  the  same  general 
character  of  dots  and  background  remains,  the 


POSITIVE  PROCESSES 


141 

bright  parts  of  the  subject  being  represented  in  the 
ink  print  by  small  black  dots  on  a white  ground, 
and  the  dark  parts  by  small  white  spots  on  a black 
ground.  The  grays  are  represented  by  intermedi- 
ate conditions  of  dots  and  ground,  and  the  ability 
to  thus  render  the  half-tones  is  responsible  for  the 
name  and  largely  responsible  also  for  the  beauty  of 
the  process.  The  number  of  these  dots  is  deter- 
mined by  the  spacing  in  the  line  screen  used  in 
front  of  the  negative,  and  the  screens  in  use  vary 
from  50  to  400  lines  per  inch.  The  best  screen  to 
use  depends  upon  the  character  of  the  subject,  the 
surface  of  the  paper  used  for  the  printing,  and  the 
mechanical  method  of  making  the  ink  impression. 


CHAPTER  VII 


LENSES 

91.  Pinhole  Images. — It  is  not  necessary  to  have 
a lense  in  order  to  take  a picture  on  a sensitive  plate. 
If  the  ordinary  lens  of  a camera  be  replaced  by  a 
very  fine  hole,  like  that  made  by  a very  fine  needle 


S 


in  a piece  of  tinfoil,  there  will  be  an  image  formed 
on  the  plate.  The  way  in  which  it  is  formed  is 
shown  in  Fig.  19.  Light  from  one  point  in  the 
field  of  view  can  get  at  only  one  point  on  the  plate, 
so  that  the  light  forms  an  image,  inverted  and  right 
left-handed,  as  in  the  case  of  the  lens  image.  The 
sharpness  of  this  image  depends  first  of  all  on  the 
size  of  the  pinhole  and  then  on  the  distance  of  the 

142 


LENSES 


143 


plate  from  the  pinhole.  At  first  sight  it  would  look 
as  if  reducing  the  size  of  this  hole  would  sharpen 
the  image  no  matter  how  small  the  hole  already 
was,  but  another  property  of  light  which  we  do 
not  ordinarily  notice  enters  to  upset  this  predic- 
tion. Along  the  edge  of  a sharp  shadow  light  does 
not  travel  in  perfectly  straight  lines  but  bends  into 
the  shadow  very  slightly,  so  slightly  in  fact  that  it 
is  very  difficult  to  detect.  Moreover,  as  it  spreads, 
light  from  the  two  sides  of  the  pinhole  have  differ- 
ent distances  to  travel  to  reach  a point  in  the  image 
out  of  the  direct  line  with  the  pinhole  and  source 
of  the  light,  so  that  the  two  waves  as  they  reach 
this  point  may  be  out  of  phase,  that  is  one  wave  may 
be  a crest  and  the  other  a trough,  and  the  addition 
of  the  two  will  result  in  no  wave.  This  takes  place 
in  such  a way  around  the  central  bright  spot  that 
there  will  be  formed  a series  of  bright  and  dark 
rings.  Fig.  20,  which  is  a photograph  of  a small 
bright  source  of  light  taken  through  a pinhole.  The 
central  bright  spot  is  much  the  brighter,  and,  going 
outward,  the  rings  decrease  rapidly  in  brightness 
till  they  disappear.^ 

As  the  pinhole  is  made  smaller  than  a cer- 
tain size  the  central  image  becomes  larger  and 

^ See,  for  example,  M.  E.  Hufford,  Physical  Rev.,  second  se- 
ries, 3,  242  (1914). 


144 


PHOTOGRAPHY 


also  the  proportion  of  the  total  light  which  is 
in  the  central  image  becomes  smaller,  so  that  the 
image  of  the  point  does  not  become  smaller  but 
really  larger  when  the  pinhole  has  passed  a certain 
size  at  which  the  definition  is  best.  The  size  of  the 
image  is  directly  proportional  to  the  distance  of  the 
plate  from  the  pinhole,  as  is  easily  seen  from  the 


geometry  in  Fig.  19.  Compared  in  brightness 
with  the  image  formed  by  a lens,  this  image  is 
exceedingly  faint,  and  the  exposure  required  may 
be  hundreds  or  thousands  of  times  that  with  the 
lens.  But  it  has  some  marked  advantages  over  an 
image  formed  by  a lens;  the  whole  picture  is  in 
focus  at  once,  foreground  and  background,  Fig.  21  ; 
page  146;  it  cannot  be  got  out  of  focus,  and  the 


Fig.  17.  Photographs  of  a narrow  slit  illuminated  from  be- 
hind with  an  arc  light.  Each  image  from  left  to  right  received 
5 times  the  exposure  of  the  preceding  one.  Series  b was  taken 
on  an  ordinary  plate,  and  series  a on  a double  coated  plate. 
See  Art.  77. 


Fig.  20.  The  image  of  a small  very  bright  source  of  light 
taken  through  an  almost  perfectly  round  pinhole  on  a plate 
about  a meter  away  from  the  pii  hole.  It  is  reproduced  the 
same  size.  See  Art.  91. 


LENSES 


145 

geometric  correspondence  between  image  and  object 
is  perfect.^ 

92.  A lens  may  be  described  as  a transparent 
medium  bounded  by  regularly  curved  surfaces,  or 
some  of  the  surfaces  may  be  planes.  The  curved 
surfaces  are  almost  invariably  spherical;  some  few 
exceedingly  expensive  telescopic  lenses  are  some- 
times varied  from  spherical  by  tedious  hand  polish- 
ing, taking  years  to  do.  All  photographic  lens  sur- 
faces are  plane  or  spherical,  see  Fig.  22.  This 
allows  of  several  different  types  of  lenses.  Fig. 
23,  falling  into  two  classes,  (a)  one  spoken  of  as 
converging  lenses  because  they  will  bring  a parallel 


beam  of  light,  like  sunlight,  to  a focus,  and  (b)  the 
other  spoken  of  as  diverging  lenses  because  a beam 
of  light  like  sunlight  will  be  spread  out  by  passage 
through  the  lens  so  that  it  will  appear  as  if  it 
spread  from  a point  on  the  side  of  the  lens  on  which 
the  light  came.  These  two  points  are  both  called 

* See  “Pinhole  Photography,”  Photominiature,  No.  27. 


146 


PHOTOGRAPHY 


foci,  the  first  a real  focus  and  the  second  an  imagi- 
nary or  virtual  focus.  The  straight  line  through 
the  centers  of  the  spheres  of  which  the  lens  surfaces 
are  a part,  is  called  the  axis  of  the  lens.  If  one 
of  the  lens  surfaces  is  a plane,  the  axis  meets  it  per- 
pendicularly while  it  also  passes  through  the  center 
of  the  sphere  of  which  the  other  surface  is  a part. 


When  a lens  is  formed  of  more  than  one  piece  of 
glass,  as  with  the  majority  of  photographic  lenses, 
the  centers  of  all  the  spheres  should  lie  on  the  one 
line;  a lens  otherwise  perfect  may  be  very  much 
impaired  in  definition  of  image  if  the  makers  of 
the  mount  have  not  been  careful  to  have  this  condi- 
tion filled. 

93.  Images. — The  method  of  formation  of  an 
image  by  a converging  lens  is  shown  in  diagram  in 
Fig.  24.  If  the  object  is  very  distant  and  on  the 
principal  axis  the  light  from  it  will  be  practi- 
cally parallel  as  it  strikes  the  lens  and  it  will  be 


Fig.  2T.  Pinhole  image.  Taken  through  a pinhole  about 
^ mm  diameter  on  a plate  5 inches  away  by  an  exposure  of  l 
minute.  Reproduced  the  same  size.  See  Art.  91. 


Fig.  28.  The  three  photographs  are  of  a book  set  on  a 
slight  slope.  In  28  the  focal  length  of  the  lens  was  5 inches 
and  the  center  of  the  liook  was  15  inches  from  the  lens. 


lenses 


147 


brought  to  a focus  at  a point  which  is  called  the 
principal  focus,  see  Fig.  25.  The  distance  of  this 
point  from  the  lens  is  called  the  focal  length  of  the 


lens.  In  this  connection  the  two  points  N and  N' 
are  important,  see  Fig.  24.  They  are  called  nodal 
points  and  are  so  placed  that  rays  of  light  entering 


Fig.  26. 


one  from  its  side  of  the  lens  will  appear  to  have 
come  from  the  other  as  it  emerges  from  the  lens. 


PHOTOGRAPHY 


148 

They  are  more  useful  and  accurate  than  tfie  older 
treatment  using  the  optical  center. 

They  have  another  property,  shown  in  Figs.  24 
and  26;  it  is  that  planes  cutting  the  axis  perpen- 


dicularly at  the  points  N and  N'  are  so  placed  that 
rays  appearing  to  strike  a point  in  the  forward 
plane,  leave  the  lens  as  if  they  came  from  a point 
in  the  rear  plane  immediately  back  of  the  first 
point.  This  makes  it  very  simple  to  follow  the 
actual  path  of  a ray  through  the  lens  and  therefore 


LENSES 


149 


also  simplifies  the  construction  of  lens  diagrams 
showing  the  formation  of  images.  In  measuring 
the  focal  length  of  a lens,  the  distance  is  measured 
from  the  nearest  nodal  point  to  the  image  of  a very 
distant  object,  the  sun  for  example.  These  points 
are  of  the  greatest  value  in  the  calculating  and  de- 
signing of  lenses.  One  other  property  deserves 
mention;  that  is  if  the  lens  be  rotated  about  either 
of  these  points  the  image  will  not  move.  It  is  about 
one  of  these  points  that  the  panoramic  camera  lens  is 
made  to  rotate  in  order  to  get  the  picture  covering  at 
least  180'.  These  points  apply  not  only  to  lenses 
formed  of  one  piece  of  glass  but  also  to  lenses 
formed  of  a number  of  pieces  and  whether  they 
are  in  contact  or  not. 

94.  Size  of  Image. — From  the  diagram  in  Fig. 
24,  it  is  very  simple  geometry  to  show  that  the 
size  of  the  image  is  to  the  size  of  the  object  as  their 
respective  distances  from  the  lens,  or  more  strictly 
from  the  nodal  points.  The  distance  of  the  image 
from  the  lens,  for  a given  object  distance,  is  deter- 
mined by  the  focal  length  of  the  lens.  Thus  if  one 
has  to  take  a picture  of  an  object  from  a fixed  dis- 
tance, the  size  of  the  picture,  that  is  of  the  image, 
can  be  varied  by  choosing  lenses  of  different  focal 
lengths,  the  longer  the  focal  length  the  larger  the 
image,  see  Fig.  27,  the  size  of  the  image  and  its 


PHOTOGRAPHY 


150 

distance  from  the  lens  being  in  all  cases  propor- 
tional. For  the  same  reasons  the  focal  lengths  of 
lenses  made  for  use  with  large  plates  are  in  general 
longer  than  those  for  use  with  small  plates.  A 
choice  of  focal  length  is  an  advantage  in  another 
case,  namely  where  to  get  the  desired  size  of  pic- 
ture, the  object  has  to  be  brought  close  to  the  lens, 
which  results  in  bad  perspective  and  in  distortion, 
the  near  parts  of  the  object  looking  too  large;  Art. 
139  and  Fig.  28.  A longer  focus  lens  enables  one  to 
move  the  camera  farther  away  from  the  object 
with  a corresponding  improvement  in  the  character 
of  picture. 

95.  Defects  in  Images  Formed  by  Lenses. — 
Aberrations. — If  one  examines  the  image  formed 
by  a lens  consisting  of  one  piece  of  glass  with  spheri- 
cal surfaces,  one  will  find  some  serious  defects,  that 
is  to  say  a serious  lack  of  correspondence  between 
the  geometrical  form  of  the  object  and  of  the  image. 
For  instance  fine  lines  in  the  object  will  be  repre- 
sented by  blurred  lines  in  the  image,  that  is  the 
image  is  not  sharp,  no  matter  how  carefully  one 
focuses.  The  middle  of  the  image  will  be  in  better 
focus  than  the  edges,  and  if  one  examines  carefully 
it  will  be  found  that  every  bright  object  is  fringed 
with  color  in  the  image.  It  will  be  necessary  to  dis- 
cuss at  least  seven  different  things  which  go  to  make 


Fig.  28a.  The  focal  length  of  the  lens  was  13  inches  and 
the  book  was  moved  away  so  as  to  make  the  central  width 
about  as  in  Fig.  28. 


Fig.  28b.  The  telephoto  lens  had  about  30  inches  equivalent 
focal  length. 


LENSES  151 

a lens  image  an  imperfect  copy  of  the  original.  They 

are 

Spherical  aberration  and  coma, 

Chromatic  aberration, 

Curvature  of  field. 

Astigmatism, 

Distortion, 

Flare, 

Unequal  illumination. 


96.  The  most  serious,  since  the  most  difficult  to 
correct,  is  spherical  aberration.  It  causes  a blur- 
ring of  the  image  and  the  explanation  is  best  made 
in  terms  of  the  diagram  Fig.  29.  The  rays  of 
light  near  the  edge  of  the  lens  are  brought  to  a 
focus  usually  nearer  to  the  lens  than  the  rays  pass- 
ing near  the  center,  the  result  being  a blurred  focal 
point;  of  course,  the  same  thing  will  happen  with 
the  image  of  every  point  in  the  object.  If  one  puts 
a diaphragm  in  front  of  the  lens  so  as  to  cut  off 
the  marginal  rays  the  image  will  become  clearer, 
the  lines  sharper  and  the  image  will  seem  to  move 
farther  from  the  lens,  at  least  the  clearest  place 


152 


PHOTOGRAPHY 


will  be  farther  from  the  lens.  But  it  helps  the  defi- 
nition, and  that  is  the  reason  one  uses  a small  dia- 
phragm when  possible.  The  diaphragm  is  best 
placed  some  distance  from  the  lens,  as  will  be  dis- 
cussed more  in  detail  under  distortion.  A form  of 
lens  in  which  spherical  aberration  is  small  is  one 


with  concentric  spherical  surfaces,  and  a number  of 
fine  lenses  are  on  this  plan.  A deep  meniscus  lens, 
see  Fig.  23  third  lens,  used  with  the  hollow  surface 
toward  the  object  has  also  a relatively  small  amount 
of  spherical  aberration. 

97.  Coma. — If  rays  coming  from  a distant  ob- 
ject placed  on  the  axis  of  the  lens,  form  such  a 
blurred  image,  it  will  be  evident  also  that  moving 
the  point  along  the  axis  toward  the  lens  will  not 
improve  the  definition  or  indeed  change  it  materi- 


lenses 


153 


ally.  If  the  point  from  which  the  rays  come  is 
moved  off  the  axis,  the  definition  is  made  even 
worse,  see  Fig.  30,  and  in  this  case  it  has  received 
the  special  name  of  ''coma!'  In  the  case  of  lenses 
for  use  in  telescopes  coma  will  not  matter  since  the 
object  is  always  on  the  axis  or  very  nearly  so,  and 
in  this  case  the  spherical  aberration  correction  be- 
comes a much  simpler  matter  and  can  be  made  with 
a high  degree  of  perfection.  But  in  photographic 
lenses,  where  both  should  be  corrected,  approximate 
corrections  have  to  suffice.  The  amount  of  spheri- 
cal aberration  in  a lens  depends  upon  the  particular 
curves  used  to  form  the  lens  surfaces  and  upon 
which  of  the  two  curves  faces  the  incident  light. 
Turning  a lens  with  the  other  face  to  the  incident 
light  often  changes  the  amount  of  spherical  aber- 
ration quite  materially.  The  amount  does  not  de- 
pend, except  as  it  affects  the  curves,  upon  the  dif- 
ferences in  thickness  at  the  center  and  at  the  edges 
of  the  lens.  The  correction  for  chromatic  aberra- 
tion does  depend  upon  this  so  that  one  correction 
need  not  interfere  with  the  other. 

98.  Chromatic  aberration  is  a very  serious  fault 
with  simple  lenses,  that  is  lenses  formed  of  one 
piece  of  glass,  and  all  photographic  lenses  worthy 
of  the  name  are  formed  of  at  least  two  pieces  of 
glass  designed  to  correct  this  fault,  and  are  called 
''achromatic  lenses.'’  The  difficulty  arises  from  the 


154 


PHOTOGRAPHY 


fact  that  the  different  colors  of  light  are  not  brought 
to  the  same  focus  by  the  simple  lens,  but  the  violet 
rays  which  are  refracted  most  are  brought  to  a 
focus  nearer  to  the  lens  than  the  red  rays,  Fig. 
31.  The  foci  for  the  other  colors  fall  in  between. 
With  a simple  lens  the  difference  may  amount  to 


1/30  of  the  focal  length  of  the  lens,  and  is  particu- 
larly bad  in  photography  because  the  focusing  is 
done  largely  with  the  yellow  light,  while  the  photo- 
graph is  taken  largely  by  the  blue,  so  that  in  the 
photograph  the  object  will  very  generally  be  out 
of  focus.  It  affects  not  only  the  sharpness  but  since 
the  size  of  the  image  depends  on  the  distance  from 
the  lens  which  is  fixed  by  the  focal  length,  the 
images  formed  by  the  different  colors  of  light  will 
be  of  different  sizes. 

99.  Achromatism. — The  approximate  correction 
is  not  a very  difficult  matter.  Glasses  differ  very 
markedly  in  the  amount  of  separation  of  the  focus 


Fig.  31. 


lenses 


155 


for  the  different  colors,  that  is  in  what  is  called 
their  dispersion.  A flint  glass  lens  has  a much 
greater  dispersion  than  a crown  glass  lens  of  the 
same  focal  length,  see  Fig.  32.  It  will  be  a sim- 
ple matter  to  make  a diverging  flint  glass  lens 
whose  dispersion  will  be  equal  in  amount  but  oppo- 


rt 


CROWN 


FUNT 


site  in  kind  to  that  of  a converging  crown  glass 
lens.  If  light  is  passed  through  one  right  after  the 
other  the  dispersion  will  be  approximately  corrected, 
but  the  convergency  of  the  crown  glass  lens  will 
more  than  offset  the  divergency  of  the  flint  and  the 
combination  lens  will  be  a converging  one  though 
of  less  convergency  than  the  crown  glass  lens  alone. 
Such  a lens  is  called  an  achromatic  lens  or  simply 
an  achromat,  see  Fig.  33. 

This  is,  however,  only  an  approximate  correction. 
If  the  series  of  points  at  which  the  different  colors 


PHOTOGRAPHY 


156 

are  focused  m the  two  lenses,  are  placed  side  by 
side,  see  Fig.  34,  it  will  be  immediately  apparent 
that  if  any  two  of  these  color  foci  are  made  to 


match,  as  for  example  in  the  violet  and  red  in  the 
figure,  then  the  others  will  not  match  exactly  but 
only  approximately.  Lenses  can  be  made  easily  so 
that  the  dispersion  of  any  pair  of  colors  will  be 


Flint 

i 

1 i 

1 

1 

i i 

1 

Crown 

BC  D E F G 

Red  Orange  'i^ilow  Greei>  Blue  ^Indigo  Violet 

Fig.  34. 


equal  and  opposite  to  that  with  the  same  pair  of 
colors  with  the  other  lens,  and  then  the  foci  for 
these  two  colors  will  be  the  same  in  the  composite 
lens  and  the  foci  for  the  other  colors  will  be  nearly 
the  same.  The  pair  chosen  for  exact  matching 
depends  on  the  use  to  which  the  lens  is  to  be  put. 
In  lenses  for  visual  work,  like  in  microscopes,  etc., 


LENSES 


157 


the  green  and  the  orange  are  usually  chosen  because 
these  along  with  the  yellow,  which  will  fall  very 
close  to  them,  are  the  colors  which  afifect  the  retina 
of  the  eye  most  strongly.  For  photographic  work 
another  pair  is  better,  namely  the  violet  and  the 
yellow,  the  yellow  being  about  midway  among  the 
colors  used  for  focusing  and  the  violet  being  about 
midway  among  those  which  actually  take  the  pic- 
ture when  using  the  ordinary  silver  bromide  plates. 
This  makes  a fairly  satisfactory  lens  for  all  ordi- 
nary work,  and  is  the  arrangement  used  in  all  ex- 
cept the  high-priced  lenses.  But  in  case  one  is 
doing  three  color  work,  this  will  not  be  sufficient 
correction  since  the  pictures,  one  in  each  of  the 
three  colors,  must  agree  exactly  in  size  and  sharp- 
ness. The  use  of  a third  lens  of  another  kind  of 
glass,  all  three  lenses  cemented  together,  will  enable 
one  to  make  three  colors  fall  exactly  at  the  same 
focus,  instead  of  two  as  with  two  glasses;  these 
three  colors  which  match  should  be  those  used  in 
three  color  work,  usually  red,  green,  and  blue. 
When  three  colors  agree  in  foci  exactly,  the  re- 
maining colors  will  of  necessity  focus  so  close  to  the 
same  point,  that  the  lens  is  practically  perfectly 
corrected  in  this  respect  and  may  be  used  for  all 
kinds  of  fine  work.  Another  plan,  and  that  used 
in  many  or  most  modern  lenses  since  it  requires 
fewer  pieces  of  glass,  applies  only  to  lenses  of  the 


PHOTOGRAPHY 


158 

doublet  type,  that  is  lenses  formed  of  two  groups 
of  pieces  of  glass  with  the  diaphragm  between  the 
groups.  A simple  achromat  of  two  pieces  of  glass, 
as  described  above.  Fig.  34,  will  make  two  colors 
come  to  the  same  focus,  and  the  lack  of  focusing 
of  the  rest,  called  secondary  dispersion,  results  from 
either  too  great  or  too  small  correction  in  these 
colors  by  the  correcting  divergent  lens.  One  pair 
of  glasses  forming  one  group  of  the  doublet  can  be 
chosen  so  that  the  correction  for  these  neglected 
colors  will  be  too  great,  and  the  other  pair  for  the 
other  group  so  that  the  correction  will  not  be  great 
enough,  and  the  result  will  be  to  make  the  correc- 
tion for  these  neglected  colors  almost  exactly  right. 
A great  variety  of  different  kinds  of  glass  is  now 
made  by  the  glass  makers,  particularly  by  Schott 
of  Jena,  and  it  is  a comparatively  simple  matter  to 
choose  glasses  from  their  lists  which  will  have  the 
properties  needed  for  the  matching  as  described 
above.  Lenses  corrected  for  three  colors  are  called 
''apochromatic.’’  In  lenses  formed  of  groups  of 
pieces  of  glass,  it  is  necessary  that  each  cemented 
group  should  be  achromatized  separately  for  the 
main  correction  at  least,  so  that  the  lens  formed  of 
these  groups  may  have  the  same  focal  lengths  for 
the  different  colors  and  at  the  same  time  form 
images  of  the  same  size. 

100.  For  most  work  it  is  desired  that  the  image  be 


LENSES 


159 


formed  on  a plane  surface  and  be  in  focus  all  over 
it  at  once.  With  a simple  lens  this  is  not  the  case, 
but  usually  the  marginal  focus  is  shorter  than  the 
central,  that  is  to  say  the  image  is  curved  concave 
toward  the  lens.  Such  a condition  is  described  as 
^^curvature  of  field!*  When  this  is  the  case  with  a 
lens,  it  is  best  usually  to  focus  for  some  point  about 


half  way  out  to  the  edge  of  the  plate,  so  that  the 
middle  and  edges  will  be  about  equally  out  of  focus 
and  the  intermediate  parts  will  be  clear.  This  will 
avoid  any  part  being  too  badly  out  of  focus  unless 
the  curvature  is  very  great.  High-priced  lenses  are 
corrected  for  this  defect  by  the  addition  of  another 
component  (piece  of  glass)  to  the  set  of  glasses. 

loi.  If  an  object  made  up  of  cross  lines  be  fo- 
cused way  out  at  one  side  of  the  plate  it  will  often 
happen  that  one  group  of  lines  will  be  in  better 
focus  than  the  other  group  at  right  angles  to  the 
first  group.  This  is  called  astigmatism.  The  cause 
of  the  trouble  is  the  unequal  focal  length  in  the 
two  planes  as  represented  in  the  diagram.  Fig. 


i6o 


PHOTOGRAPHY 


35.  It  is  a serious  defect  in  all  cheap  lenses  and  in 
the  more  expensive  lenses  is  difficult  to  correct. 
Lenses  in  which  astigmatism,  spherical  and  chro- 
matic aberration  are  corrected  will  usually  be  com- 
posed of  three  glasses  for  each  group,  and  the  group 
is  usually  duplicated,  forming  a doublet  lens,  the 
most  common  photographic  type. 


Object 


r" 

- — 

fn] 

1 

L 

1 

1 

1 

1 

- 

— 1 — 
-I- 
1 

■“I 

1 

1 



_ JL  - 

L-j 

Imaged) 
Fig.  36. 


1 

1 

1 

1 

1 

1 

1 

I-- 

1 

1 

_r- 

1 

1 

“ 1 
1 

1 

1 

1 

1 

U4r« 

sri 

Inf)cige(b) 


102.  If  an  image  of  a set  of  lines  crossing  at 
right  angles  be  formed  by  a single  glass  lens,  it 
will  be  easily  seen  that  the  lines  in  the  image  are 
not  straight,  although  they  are  in  the  object,  see 
Fig.  36.  This  lack  of  correspondence  in  shape 
between  the  object  and  the  image  is  called  Distor- 
tion. When  a simple  lens  is  used  without  any  dia- 
phragm, that  is  at  what  is  called  full  aperture,  it 
usually  shows  distortion  which  is  directly  depend- 
ent on  the  spherical  aberration  of  the  lens.  The 
use  of  a diaphragm  by  limiting  the  rays  going  to 
any  point  in  the  image,  to  a restricted  area  of  the 


Fig.  37.  Pin  Cushion  Distortion.  Taken  with  the  front 
component  of  a fine  doublet  lens  with  the  diaphragm  behind  it. 


Fig.  37a.  Barrel  Distortion.  Taken  from  the  same  place 
and  with  the  same  component  as  Fig.  37  but  with  the  dia- 
phragm in  front  of  tiie  lens  and  the  lens  turned  around.  Beside 
the  curvature  of  what  are  straight  lines  in  the  object,  note  the 
difference  in  size  of  the  two  pictures.  The  black  line  frame  is 
the  same  size  in  the  two  original  prints. 


Fig.  47.  Microphotograph  taken  by  H.  E.  Ives  of  a cross  sec- 
tion of  a Lipmann  film  and  reproduced  here  through  his  kind- 
ness. 


LENSES 


i6i 


lens,  modifies  the  distortion  greatly.  In  case  the 
distortion  is  barrel-shaped,  as  in  Fig.  36  (a),  a dia- 
phragm in  front  of  the  lens  exaggerates  it  greatly, 
while  pin  cushion  distortion,  Fig.  36  (b),  is  exag- 
gerated by  the  diaphragm  behind  the  lens.  The  size 
as  well  as  the  shape  of  the  image  is  affected  as  will 
be  seen  readily  in  Fig.  37. 


The  obvious  remedy  is  to  use  two  lenses  placed 
on  opposite  sides  of  the  diaphragm  so  that  their  dis- 
tortions will  be  of  opposite  kinds,  and  so  neutralize 
each  other.  Largely  on  this  account  most  photo- 
graphic lenses  are  of  this  doublet  pattern.  The 
rapid  rectilinear  is  the  commonest  lens  of  this  type. 
Fig.  38.  It  has  two  simple  achromatic  lenses  with 
similar  faces  toward  the  diaphragm  and  separated 
by  a short  distance  with  the  diaphragm  half-way 
between  them. 

103.  If  one  had  a lens  formed  of  several  pieces  of 
glass,  especially  if  they  are  not  cemented  together. 


PHOTOGRAPHY 


162 

and  used  it  with  a very  small  diaphragm,  sometimes 
there  will  be  formed  in  the  picture  lights  and  shades 
which  have  nothing  to  do  with  the  object  pictured. 
The  commonest  case  is  a bright  spot  right  at  the 
center  of  the  picture.  It  is  still  more  likely  to  occur 


if  the  sunlight  falls  on  the  lens.  It  is  described  as 
''Flare''  or  "Ghost"  and  is  due  to  reflections  of  light 
back  and  forth  between  the  different  surfaces  of  the 
lens,  ultimately  some  of  it  getting  to  the  plate,  but 
by  the  time  it  does  so  it  will  be  far  from  the  place 
it  rightly  occupies  in  the  picture,  see  Fig.  39.  No 
good  lens  should  show  it  in  any  ordinary  work. 

The  proportion  of  the  light  approaching  the  lens 
which  actually  appears  in  the  expected  place  in  the 
image  lies,^  between  55  and  92%,  so  that  it 
varies  materially  in  different  lenses,  and  the  loss  in 
some  lenses  is  serious.  The  loss  is  due  to  reflection 
at  the  various  surfaces  and  to  absorption  in  the 

®P.  G.  Nutting,  Astrop'hysical  Journal,  40,  33  (1914). 


lenses  163 

glass.  Poor  polish  of  the  surfaces  and  bubbles  in 
the  glass  also  increase  the  loss. 

104.  The  remaining  difficulty  is  that  of  unequal 
illumination,  and  Fig.  40  will  show  some  of  the 
reasons  for  it.  The  width  of  the  pencil  of  light 
going  to  different  parts  of  the  plate  is  cut  down  by 


the  lens  barrel  when  it  makes  an  angle  with  the 
axis.  For  this  reason,  lenses  which  are  made  to 
cover  a large  plate  as  compared  with  their  focal 
lengths,  that  is,  lenses  taking  in  a big  angle  of  view, 
are  made  with  the  lens  barrel  as  short  as  possible. 
Even  in  case  the  beam  is  not  cut  down  by  the  bar- 
rel, it  will  be  cut  by  the  diaphragm  not  presenting 
the  same  area  in  its  direction  as  it  does  along  the 
axis.  Added  to  this,  the  light  has  to  travel  a greater 
distance  to  the  plate,  and  therefore  has  to  cover  a 


PHOTOGRAPHY 


greater  area;  moreover,  it  will  strike  the  plate  at 
an  angle  not  a right  angle,  so  that  again  it  will 
have  a greater  surface  to  cover.  All  these  things 


Fig.  41, 


lenses 


165 

add  to  make  it  quite  a serious  matter  to  get  suffi- 
ciently equal  lighting  towards  the  sides  of  the  plate. 
Fortunately  it  takes  considerable  falling  off  to  be 
easily  observed  in  the  print.  The  ordinary  lens 
takes  in  an  angle  of  view  of  35°  to  60°  while 
some  of  the  ordinary  wide  angle  lenses  will  take  in 
80°  and  the  extreme  wide  angle  may  cover  135°, 
Fig.  41.  The  narrow  to  medium  angle  lenses  give 
pictures  in  which  the  perspective  is  most  pleasing. 
In  case  one  is  forced  to  take  the  picture  from  a near 
point  of  view,  a wide  angle  is  necessary  in  order  to 
get  the  whole  subject  on  the  plate.  In  extreme 
cases,  such  as  photographing  interiors,  extreme 
wide  angle  lenses  are  necessary,  but  the  perspective 
is  exaggerated  and  is  not  pleasant. 

105.  Depth  of  Focus. — In  nearly  all  picture 
work  the  object  to  be  taken  is  not  all  at  the  same 
distance  from  the  lens,  and  the  picture  is  a projec- 
tion of  three-dimensional  subjects  onto  the  two-di- 
mensional plate.  For  every  distance  of  the  object 
the  lens  has  a distinct  position  of  the  focus,  so  that 
if  the  lens  be  focused  for  some  part  of  the  object 
the  remainder,  being  at  different  distances,  will  not 
be  in  as  good  focus.  How  bad  the  focus  will  be  in 
the  rest  of  the  picture  will  depend  on  several  things, 
the  actual  differences  in  the  distances  of  the  parts 
considered,  the  focal  length  of  the  lens,  and  the  size 
of  the  diaphragm.  The  first  usually  has  to  be  taken 


PHOTOGRAPHY 


i66 

as  it  occurs  but  where  there  is  any  choice  in  the 
matter  it  will  always  improve  the  definition  to  have 
the  objects  as  near  together  as  possible.  The  actual 
amount  of  blurring  which  the  eye  will  not  detect 
will  be  fairly  constant,  that  is,  the  image  of  a point 


Fig.  42. 


can  be  widened  out  into  a circle  of  a definite  size 
before  we  object  to  it  as  being  blurred.  It  will  be 
evident  that  with  a short  focus  lens  the  difference 
in  the  object  distances  which  will  produce  this  al- 
lowable blurring  will  be  much  greater  than  with  a 
longer  focus  lens.  For  this  reason  most  of  the 
fixed-focus  cameras,  where  the  lens  is  set  for  an 
object  at  a long  distance  away,  use  short  focus 


LENSES 


167 

lenses  and  hence  small  plates,  so  that  the  whole  field 
up  to  a few  feet  away  will  be  in  satisfactory  focus. 
With  lenses  up  to  3 or  3^4  inch  focal  length  this 
does  fairly  well  as  long  as  the  object  is  12  or  15 
feet  away,  but  when  for  the  sake  of  portraits  of 
visible  size  this  distance  is  lessened,  then  the  blur- 
ring becomes  pronounced.  The  effect  of  the  dia- 
phragm also  needs  some  consideration.  The  dia- 
gram, Fig.  42,  shows  that  for  a given  amount  of 
blurring  the  allowable  difference  in  distance  of  the 
objects  will  be  greater  for  a small  diaphragm  than 
for  a larger  one.  This  so-called  ''depth  of  focus’’ 
is  much  greater  for  the  small  diaphragm.  When 
one  goes  to  the  very  rapid  lenses  where  the  size  of 
the  diaphragm  allowable  is  very  great,  up  to  ^ or 
1/3  of  the  focal  length,  this  blurring  of  objects  even 
slightly  out  of  the  correct  focal  distance  becomes 
very  pronounced.  It  is  painfully  evident  in  pictures 
of  rapidly  moving  objects  (reflex  camera  work), 
where  the  diaphragm  has  to  be  very  large  to  get 
sufficient  light  in  the  very  short  time  which  must 
be  used  so  that  the  object  may  not  move  enough 
to  show. 

106.  When  one  examines  a lens  maker’s  catalog 
he  will  see  constant  reference  to  the  speed  of  the 
different  lenses.  Other  things  being  equal,  that  is 
the  speed  of  the  plate,  brightness  of  the  day,  size  of 
the  plate  and  focal  length  of  lens,  that  lens  which 


i68 


PHOTOGRAPHY 


will  give  the  satisfactory  definition  with  the  larger 
stop  will  allow  of  the  picture  being  taken  in  the 
least  time,  and  is  hence  spoken  of  as  the  fastest 
lens.  Lenses  of  different  focal  length  may  be  com- 


Fig.  43.  Convertible  Lens. 

pared  if  the  size  of  the  diaphragm  is  measured  in 
terms  of  the  focal  length,  that  is  in  the  P numbers. 
So  that  the  speed  of  the  lens  is  usually  spoken  of 
in  terms  of  the  largest  stop  it  will  allow  for  satis- 


Fig.  44.  Petzval’s  Portrait  Lens. 


factory  definition  and  be  referred  to  by  the  numbers 
of  the  '‘F’  or  scale. 

107.  Lens  Types. — The  rapid  rectilinear,  Fig. 
38,  page  1 61,  has  already  been  mentioned  as  the 
important  lens  in  the  medium-priced  cameras,  and 


LENSES 


169 

of  course  there  are  all  different  grades  of  rapid 
rectilinear  lenses,  depending  on  the  glass  used  for 
the  different  components  and  the  care  with  which 
they  are  finished  and  mounted.  The  cheap  cameras 
have  single  achromatic  lenses,  or  in  the  cheapest 
kinds  a single  glass  lens.  Of  these  the  deep  menis- 
cus type  is  the  best,  having  the  least  spherical  aber- 
ration. 

Some  of  the  expensive  doublet  lenses  have  the 
different  components  of  different  focal  lengths  and 
even  of  different  diameter.  Fig.  43.  Each  com- 
ponent is  corrected  as  far  as  possible  so  that  either 
one  may  be  removed  from  the  tube,  thus  providing 
the  possibility  of  three  different  focal  lengths  for 
different  work,  and  hence  they  are  called  converti- 
ble lenses. 

The  Petzval  portrait  lens.  Fig.  44,  has  been 
such  an  important  lens  in  the  past  that  it  deserves 
mention,  but  it  is  not  as  good  for  its  own  special 
work  as  some  of  the  more  modern  lenses,  while  it 
is  more  expensive  to  make  and  so  is  being  gradually 
displaced,  but  the  professional  photographers  still 
use  a good  many  of  them.  It  is  of  the  doublet 
type,  but  the  spherical  aberration  is  corrected  by  a 
meniscus  component  separated  by  an  air  space  from 
the  next  member.  By  varying  the  air  space,  the 
spherical  aberration  can  be  varied  at  will  for  soft 
or  for  sharp  definition. 


170 


PHOTOGRAPHY 


Some  good  modern  doublets  are  the  Zeiss  ^ ^'Tes- 
sar/’  Goerz  ® ''Dagor/’  Cooke  ® Anastigmat, 
Voightlander  ‘‘Heliar/’  Harris  ® ''Euryplan/' 

108.  Telephoto  Lens. — There  is  yet  one  lens 
which  requires  some  mention.  It  is  the  telephoto 
combination.  The  purpose  is  to  enlarge  the  size 
of  the  picture  without  the  great  extension  of 
the  bellows  required  when  the  enlarging  is  done 


by  simply  increasing  the  focal  length  of  the  lens, 
which  moreover  leads  to  clumsy  apparatus  and 
requires  a very  steady  support.  The  idea  of  the 
telephoto  lens  will  be  best  obtained  from  the  dia- 
gram, Fig.  45.  It  consists  of  the  usual  high  class 
lens  of  the  converging  type,  and  at  some  distance 
behind  it  is  placed  a diverging  lens.  As  shown 
in  the  diagram  this  second  lens  reduces  the 
convergency  of  the  rays  so  that  they  will  strike 
the  plate  as  if  they  had  come  through  a very  long 

^ Bausch  & Lomb  Optical  Co.,  Rochester,  New  York. 

® C.  P.  Goerz  American  Optical  Co.,  317  E.  34th  St.,  New 
York. 

® Taylor,  Taylor  and  Hobson,  1133  Broadway,  New  York. 

^ Voightlander  & Son,  A.  G.,  225  Fifth  Avenue,  New  York. 

® Ralph  Harris  & Co.,  26  Bromfield  St.,  Boston,  Mass. 


lenses 


I7I 

focus  lens.  By  varying  the  distance  between  the 
two  lenses  the  apparent  focal  length  changes  and 
with  it  the  size  of  the  picture.  The  greatest  sepa- 
ration is  when  the  diverging  lens  is  at  the  focus  of 
the  converging,  when  the  magnification  becomes  in- 
finite, and  in  practice  the  separation  is  always  less 
than  this.  In  the  first  arrangements  of  this  combi- 
nation put  out  by  the  lens  makers  the  highest  pos- 
sible magnification  was  sought,  and  this  was  not 
always  needed  and  was  frequently  the  cause  of  poor 
results.  So  that  now  the  magnifications  usually  run 
from  four  to  ten  or  twelve.  The  camera  requires 
steady  support  and  the  exposures  have  to  be  much 
longer  than  with  the  usual  lens.  If  the  arrange- 
ments are  good  the  picture  is  better  in  the  way  of 
detail  than  can  be  obtained  by  enlarging  a pic- 
ture taken  with  an  ordinary  lens.®  The  diverging 
lens  part  of  the  combination  is  not  very  expensive, 
and  it  is  used  with  the  high  grade  general  purpose 
lens. 

There  are  a number  of  very  readable  books 
on  lenses,  for  example : 

C.  L.  Johnson,  Photographic  Optics  and  Color 
Photography, 

J.  T.  Taylor,  Optics  of  Photography  and  Photo- 
graphic Lenses, 

*See  ^Telephotography,”  Photominiature,  No.  26. 


172 


PHOTOGRAPHY 


Beck,  Photographic  Lenses, 

While  a more  advanced  treatment  may  be  found 
in: 

Otto  Lumner,  Photographic  Optics,  translated  by 
S.  P.  Thompson. 


CHAPTER  VIII 


COLOR  PHOTOGRAPHY 


109.  There  are  a great  many  reasonably  work- 
able methods  of  taking  photographs  of  objects 
where  the  photograph  will  show  the  colors  as  well 
as  the  geometrical  shape  and  the  light  and  shade. 
This  is  a different  problem  from  the  photography 
of  colored  objects,  where  the  purpose  was  to  render 
the  visual  light  and  shade  of  the  colors  in  one  color, 
e.g.,  gray.  Now  we  desire  to  include  the  color  itself 
in  the  finished  picture. 

These  processes  may  be  grouped  immediately  into  ^ 
two  classes : I.  those  in  which  no  pigments  are^u^bd 
to  produce  the  colored  picture,  of  which  the  Lipp-  ^ 
mann  process  is  the  only  one.-^d  II.  those  usually 
known  as  three-color  processes  which  use  pigment 
to  give  color  to  the  print,  which  includes  a great 
number  of  methods.  This  last  class  may  be  subdi- 
vided again: 


1.  Those  using  three  negatives 

(A)  Addition  positives 

(B)  Subtraction  positives 

2.  Those  using  a mosaic  three-colored  screen 

3.  One  using  the  bleaching  by  light  of  some  dyes. 

173 


174 


PHOTOGRAPHY 


C1.ASS  I.  Lippmann  Process 


no.  To  understand  the  Lippmann  process  it  will 
be  necessary  to  review  something  of  the  theory  of 
light.  The  generally  accepted  theory  is  that  light 
is  a wave  motion  in  an  elastic  medium,  the  wave 
motion  being  of  the  transverse  type,  the  velocity 
being  very  great  and  the  wave  lengths  very  short, 

^ fev’"  /~n  ■»  o o M o /*\-C  T -f  .'vn o 


of  the  same  wave  length  go  in  opposite  directions 
in  the  same  medium  they  will  interfere  in  such  a 
way  as  to  set  up  standing  waves,  that  is,  places 
where  the  two  waves  destroy  each  other,  and  places 
where  they  help  each  other,  Fig.  46.  One  train 
of  waves  may  be  most  easily  formed  by  the  reflec- 


GLASS 


FILM 


Fig.  46.  Lipmann  Process. 


COLOR  PHOTOGRAPHY 


175 


tion  of  the  first  train,  and  there  are  many  exam- 
ples of  standing  waves  by  such  an  arrangement. 
Light  being  a wave  motion  should  allow  of  the 
formation  of  standing  waves.  That  is  to  say,  if  a 
beam  of  light  be  allowed  to  fall  perpendicularly  on 
a mirror,  the  two  beams  passing  in  opposite  direc- 
tions on  the  same  path  should  set  up  standing 
waves,  that  is  to  say,  places  of  no  light  and  places 
of  increased  light  following  each  other  regularly 
along  this  path.  Suppose  now  that  the  path  of  the 
light  immediately  above  the  mirror  be  filled  with  a 
semi-transparent  sensitive  film,  then  the  standing 
waves  should  be  formed  in  it,  and  the  places-of-no- 
wave  should  not  be  acted  on  by  any  light,  and  the  in- 
termediate places  should  be  strongly  acted  on. 
If  this  film  be  developed  it  should  show  a series 
of  layers  of  deposit  of  silver  where  the  light  acted, 
with  intermediate  places  where  the  silver  will  not 
be  reduced.  The  unaffected  silver  haloid  can  be 
dissolved  out  by  hypo  leaving  this  series  of  layers 
of  reduced  silver.  If  a thin  strip  is  cut  off  the  edge 
of  any  piece  of  the  film  and  then  examined  under 
the  microscope,  it  will  be  seen  to  be  filled  with  a 
series  of  opaque  layers  separated  by  transparent 
portions.  Fig.  47,  page  160,  the  layers  running  par- 
allel to  the  surface  of  the  film,  and  the  number  of 
such  layers  may  run  up  into  the  hundreds.^ 

Hves,  Astrophysical  Journal,  27,  325  (1908). 


PHOTOGRAPHY 


176 

Let  us  follow  the  argument  a little  farther.  If 
one  holds  this  layer  film  up  in  front  of  one’s  face 
with  the  light  coming  past  the  head  so  as  to  fall  on 
the  many  layered  film  perpendicularly,  each  is  near- 
ly transparent  but  will  reflect  some  light  so  that 
the  light  which  is  reflected  to  the  eye  will  be  the 
sum  of  the  reflections  from  these  different  layers. 


Fig.  48.  Diagram  showing  how  the  light  reflected  from  the 
different  layers  in  the  Lippmann  film,  returns  along  one  line 
and  therefore  interferes  destructively  unless  the  wave  length  of 
the  light  is  twice  the  distance  between  the  layers. 

The  light  reflected  from  two  layers  will  add  to- 
gether to  produce  more  light  only  if  the  two  waves 
are  in  phase,  and  this  will  only  be  the  case  when 
the  distance  between  the  layers  is  half  a wave  length 
of  the  light  used.  In  consequence  the  colors  which 
are  not  of  the  right  length  of  wave  to  agree  with 
the  distance  between  the  layers  will  be  destroyed 


COLOR  PHOTOGRAPHY 


177 


by  interference,  while  the  light  which  agrees  with 
this  length  will  be  increased  by  each  successive  ad- 
dition of  more  light  by  lower  layers,  Fig.  48. 
The  space  between  the  layers  was  determined  by 
the  light  which  fell  upon  the  film,  and  therefore  the 
film  will  look  this  same  color  when  viewed  perpen- 
dicularly afterward. 

Lippmann  argued  very  much  like  this  above,  and 
it  was  looked  on  as  a great  triumph  of  the  wave 
theory  that  it  worked  almost  exactly  as  expected. 
There  are  a number  of  experimental  details  and 
difficulties  that  the  theory  does  not  take  any  ac- 
count of.  The  film  has  to  be  very  thin  so  that  it 
will  not  be  practically  opaque  to  the  light  falling 
upon  it,  so  that  the  ordinary  plates  will  not  serve 
but  ones  with  specially  thin  films  have  to  be  made. 
The  reflector  must  be  in  intimate  contact  with  the 
film,  which  is  usually  accomplished  by  exposing 
from  the  glass  side  and  making  the  film  the  side  of 
a box  containing  mercury.  The  film  is  very  thin 
and  develops  very  rapidly,  and  must  not  be  devel- 
oped very  long  or  it  will  be  much  too  dense.  If  it 
is  treated  with  hypo  the  loss  of  silver  from  the 
undeveloped  layers  will  bring  the  layers  of  reduced 
silver  a little  closer  together,  and  the  colors  will  be 
shifted  a little. 

The  exposure  is  made  in  the  camera  with  the 
mercury  against  the  sensitive  film,  and  the  expo- 


PHOTOGRAPHY 


178 

sures  have  to  be  long  on  account  of  the  fine  grained 
(and  therefore  slow)  emulsion  which  has  to  be 
used.  It  is  a difficult  process  for  a patient,  skillful 
worker.  Pictures  of  such  a subject  as  the  spectrum, 
where  the  colors  are  formed  of  one  wave  length, 
are  reproduced  so  that  one  has  no  difficulty  in  be- 
lieving that  the  spectrum  is  actually  seen.  But 
when  one  asks  the  film  to  reproduce  the  ordinary 
colors  of  colored  objects  where  the  colors  are  mix- 
tures of  a lot  of  wave  lengths  the  rendering  is  not 
nearly  so  good,  but  still  the  pictures  are  well  worth 
while.  White,  being  a mixture  of  all  the  colors,  is 
the  most  difficult  to  render  and  is  the  stumbling 
block  in  most  cases.  The  method  is  of  immense 
scientific  interest  but  is  too  difficult  for  ordinary 
use.^ 

CnAss  II.  Thri:e-Color  Proce:sse:s 

III.  All  of  the  processes  for  the  making  of  pic- 
tures in  colors  by  the  use  of  pigments  in  a me- 
chanical way  are  based  on  observations  first  made 
by  Maxwell.  The  result  of  these  observations  can 
be  summarized  in  the  statement  that  any  color  as 
perceived  by  the  eye  may  be  imitated  by  mixing 

*In  the  British  Journal  Color  Supplement  for  November  4, 
1910,  will  be  found  detailed  directions  for  the  Zeiss  apparatus 
and  Jahr  plates  for  working  this  process. 


COLOR  PHOTOGRAPHY 


179 


in  proportions  suitable  for  the  particular  case  three 
primary  colors,  orange-red,  green,  and  blue-violet. 
There  is  some  latitude  in  the  choice  of  these  three 
colors,  but  if  the  visible  spectrum  be  divided  into 
three  about  equal  parts  each  will  be  about  the  color 
required.  To  make  use  of  this  imitation  of  the 
original  colors,  it  is  necessary  to  make  three  pic- 
tures in  the  three  primary  colors  and  superpose 
them.  The  methods  for  doing  this  differ  in  cases 
(l),  (2),  and  (3),  page  173. 

1 12.  Case  I. — In  both  modifications  of  the  three- 
negative process,  three  pictures  are  taken  on  color- 
sensitive  plates,  each  through  a screen  colored  with 
one  of  the  three  primary  colors.  That  is,  the  nega- 
tive taken  through  the  red  screen  will  have  a de- 
posit of  silver  wherever  the  original  subject  re- 
flected red  light.  This  will  mean  not  only  the  red 
objects,  but  all  objects  of  mixed  colors  which  in- 
clude some  red  in  the  mixture,  and  all  objects  re- 
flecting white  light  which  most  objects  do  to  some 
extent.  So  also  for  the  other  negatives,  that  is 
each  one  will  contain  a lot  of  detail,  and  unless 
there  are  some  markedly  colored  objects  there  will 
be  trouble  in  distinguishing  the  three  negatives. 
These  negatives  are  called  the  ''color  records,'’  and 
there  are  a variety  of  methods  for  translating  the 
records  again  into  color. 

1 13.  (a)  The  additive  method.  This  is  the  most 


i8o 


PHOTOGRAPHY 


direct  and  simplest  in  principle  of  the  methods. 
Three  positives  are  made  from  the  three  negatives 
and  each  provided  with  a screen  colored  like  the 
screen  through  which  the  negative  was  taken.  These 
three  positives  are  then  placed  in  three  different 
projecting  lanterns,  each  positive  with  the  original 
taking  screen  and  the  three  pictures  matched  care- 
fully on  the  screen.  This  will  reproduce  the  orig- 
inal beautifully,  but  requires  an  expensive  outfit  and 
takes  time  to  change  from  one  picture  to  another. 
A simpler  and  cheaper  arrangement  is  Ives  Kroms- 
kop/  in  which  an  arrangement  of  transparent 
glass  mirrors  allows  of  seeing  the  three-colored  pic- 
tures superposed,  thus  giving  the  mixed  colors  of 
the  original,  see  Fig.  49.  The  apparatus  may  also 
be  arranged  for  projection  with  one  lantern. 

1 14.  (b)  The  subtractive  method.  This  is  less 

simple  in  principle  but  more  usable  practically.  In 
this  group  are  a number  of  distinct  processes  all 
requiring  the  same  three-color  negatives.  Of  these 
the  important  ones  are  (a)  Carbon  (including  San- 
ger-Shepherd,^  Autotype,  Rotary,  Tripack,^),  (b) 

^ For  an  excellent  working  description  see  Konig-Wall  Nat- 
ural Color  Photography. 

^ Supplies  may  be  obtained  from  Sanger-Shepherd  & Co., 
Ltd.,  5 Gray’s  Inn  Passage,  Red  Lion  St.,  Holborn,  London  W. 
C.,  England.  . 

® Supplies  and  directions  may  be  obtained  from  Hess-Ives’ 
Corporation,  1201  Race  St.,  Philadelphia,  Pa. 


COLOR  PHOTOGRAPHY 


i8i 


Pinatype,  (c)  Traube  Iodide,  (d)  Printing  press. 
Three-colored  prints  are  made  one  from  each  of  the 
three  negatives  and  superposed.  The  printing  col- 


' Fig.  49.  Ives  Kromskop. 


ors  used  require  some  explanation.  If  one  printed, 
for  example,  the  red  sensation  negative  on  red 
material,  then  since  every  red  object  leaves  a de- 
posit on  the  negative,  all  the  red  objects  will  be 
white  in  the  red  print  and  all  the  rest  of  the  picture 
which  included  no  red  will  have  red  all  over  it. 


i82 


PHOTOGRAPHY 


Similarly  for  the  other  three  colors.  So  that  this 
evidently  will  not  do;  as  a matter  of  fact  every 
object  would  appear  in  its  complementary  color. 
But  if  the  negatives  be  printed  each  in  the  comple- 
mentary color  to  its  taking  screen,  then  the  picture 
will  come  out  right.  Let  us  represent  the  spectrum 
by  a straight  line  thus,  r o y g b v;  then  the  print- 
ing colors  and  screens  will  be  as  follows,  the  hori- 
zontal lines  meaning  that  these  colors  of  light  are 
present. 


Screen 

Printing  color 

RED 

GREEN 

BLUE-VIOLET 

r 0 

y g 

b V 

r 0 

y g 

b V 

GREEN- 
BLUE 
RED  (pur- 
plish) 
YELLOW 

Now  consider  a red  object  in  the  picture.  In  the 
red  sensation  negative  there  will  be  a deposit  at  this 
place,  and  therefore  this  object  will  be  represented 
by  a clear  place  in  the  print.  In  both  the  other 
prints  there  will  be  clear  places  in  the  negatives  and 
therefore  deposit  in  the  print.  The  only  color  not 
absorbed  by  either  of  these  two  deposits  is  the  red, 
either  one  or  the  other  deposit  absorbing  the  rest 
of  the  spectrum.  That  is,  in  the  print  this  place  is 


COLOR  PHOTOGRAPHY  183 

red.  So  also  for  the  other  colored  objects  and  for 
the  mixed  colored  objects. 

115*  (a)  Just  a few  words  as  to  how  the  different 
colored  prints  are  made  in  the  different  methods. 
The  carbon  process  of  printing  has  already  been 
described,  Art.  90.  A number  of  manufacturers  ® 
are  making  tissue  in  the  three  colors  required  for 
these  prints.  From  the  previous  description  it  will 
be  easily  seen  how  readily  the  three  prints  can  be 
superposed.  To  avoid  the  double  transfer  one  firm 
makes  the  tissue  on  a transparent  celluloid  sup- 
port, and  the  printing  is  done  through  the  support. 
In  this  case  the  insoluble  part  of  the  gelatine  will 
be  next  the  support,  and  the  soluble  part  may  be 
washed  off  without  any  preliminary  transfer.  To 
superpose  the  three  pictures  exactly  is  not  an  easy 
operation,  and  it  requires  a good  deal  of  care  to 
keep  the  three  pictures  of  the  same  size. 

1 16.  (b)  The  Pinatype  process  depends  on  the 

different  actions  of  soluble  and  insoluble  gelatine 
when  treated  with  certain  dyes.  When  a film  of 
soluble  gelatine  is  dipped  in  a solution  of  one  of 
these  dyes  the  dye  is  absorbed  by  the  gelatine  and 

"Autotype  supplies  for  all  kinds  of  carbon  printing  may  be 
obtained  from  George  Murphy,  57  E.  9th  St.,  New  York. 

’ Supplies,  with  detailed  directions,  may  be  obtained  from 
Meister,  Lucius  & Briining,  Hoechst  am  Main,  Germany,  or 
from  the  American  agents,  Farbwerke-Hoechst,  P.  O.  Box  753, 
New  York. 


PHOTOGRAPHY 


184 

stains  it  deeply.  The  insoluble  gelatine  will  not 
absorb  the  dye.  A film  of  clear,  soft  gelatine  is 
sensitized  with  chromate  and  then  exposed  under  a 
positive  made  from,  say,  the  original  red  sensation 
negative.  The  gelatine  film  is  washed  free  of  chro- 
mate and  then  soaked  in  the  dye  solution  (green- 
blue).  The  dye  will  be  absorbed  by  the  soluble  gela- 
tine, and  not  by  the  insoluble,  and  the  detail  of 
the  picture  shows  clearly.  The  other  two  colors  are 
printed  in  the  same  way;  the  three  are  superposed 
and  looked  through,  and  will  show  the  colors  of  the 
original  object  if  the  depth  of  color  of  each  of  the 
three  prints  is  right.  This,  of  course,  requires  thin, 
transparent  supports  for  each  print.  By  a simple 
extension  the  prints  may  be  duplicated  many  times 
and  on  a paper  support.  One  of  the  above  dyed 
prints  is  dyed  very  deeply  and  then  rubbed  into  close 
contact  with  a wet,  soluble-gelatine  surface.  The 
dye  transfers  itself  slowly  to  the  new  gelatine, 
which  after  a few  minutes  will  show  the  picture 
clearly.  Each  of  the  other  colors  is  transferred  in 
the  same  way  to  the  same  gelatine  film,  which  wih 
then  show  the  picture  in  the  original  colors.  The 
difficult  points  are  to  get  all  three  pictures  correctly 
superposed  and  printed  to  the  right  depth  of  color. 
As  might  be  expected  from  the  process  the  prints 
are  not  very  sharp  but  are  not  unpleasantly  fuzzy 
and  may  be  duplicated  almost  indefinitely. 


COLOR  PHOTOGRAPHY  185 

1 17.  (c)  All  of  you  will  recall  that  dry  plates  are 

made  sensitive  to  the  spectrum  by  dipping  the  dry 
plates  in  a solution  of  a dye.  The  particles  of  silver 
bromide  absorb  a small  quantity  of  the  dye.  Silver 
iodide  will  absorb  large  quantities  of  many  dyes 
but,  unlike  silver  bromide,  the  dye  has  but  little 
effect  on  the  color  sensitiveness.  Traube  has  made 
this  property  of  silver  iodide  the  foundation  of  a 
process  for  printing  in  one  or  more  colors.®  The 
silver  in  an  ordinary  positive  on  paper  or  other  sup- 
port is  changed  over  to  silver  iodide  by  immersing 
in  a solution  of  potassium  ferricyanide  and  potas- 
sium iodide  (compare  the  ferricyanide  reducer). 
This  bleached  positive  is  then  immersed  in  a solu- 
tion of  the  dye  which  stains  both  iodide  and  gelatine 
but  washes  readily  out  of  the  gelatine.  The  next 
step  is  to  dissolve  out  the  silver  iodide  by  a solution 
of  hypo,  which  also  has  to  contain  a substance  to 
render  the  dye  insoluble,  a mordant,  such  as  tannic 
acid.  The  silver  image  is  thus  replaced  by  a dye 
image  whose  color  and  density  may  be  regulated 
almost  at  will  by  the  choice  of  dye  and  density  of 
the  original  silver  positive.  The  modification  nec- 
essary to  make  three-color  prints  is  slight.  The  re- 
quired three-color  prints  on  transparent  supports 
are  made  from  the  red,  green,  and  blue  sensation 

* Supplies  and  detailed  directions  may  be  obtained  from  Otto 
Perutz,  Munchen,  Dachauerstrasse  50,  Germany. 


1 86 


PHOTOGRAPHY 


negatives  and  superposed.  The  process  offers  ad- 
vantages to  the  amateur  in  its  similarity  to  the 
customary  process  and  its  ease  of  control. 

118.  (d)  The  printing  press  using  colored  print- 
ing inks  occupies  first  rank  when  measured  by  the 
number  of  pictures  produced.  In  the  usual  three- 
color  process,  the  three  negatives  described  above 
are  used  to  make  three  printing  blocks,  and  each  of 
these  is  used  to  impress  its  appropriate  color  on 
the  print.  In  skillful  hands  the  results  are  ex- 
ceedingly fine.  For  the  making  of  one  copy  it  would 
be  a very  expensive  process,  but  when  the  cost  of 
the  blocks  is  charged  against  some  thousands  of 
copies,  the  cost  per  copy  is  not  large.  It  is  a diffi- 
cult process  requiring  extensive  equipment  and 
highly  skilled  workers,  so  that  it  is  not  at  all  suit- 
able for  general  use. 

1 19.  Case  2. — Mosaic  Screen  Process.® — At  the 
present  time  these  are  the  most  popular  with  the 
ordinary  worker.  The  method  was  first  worked 
out  by  Professor  Joly  of  Dublin.  A number  of 
makes  have  been  tried  commercially — ^Joly,  Mac- 
Donough,  Warner-Powrie,  Thames,  Krayn,  Omni- 
color, Autochrome,  Dufay,  Leto,  Paget,^®  the  last 

•Mees,  C.  E.  K.,  “History  of  Color  Screen  Photography,” 
British  Jour.  Sup.^  1908,  p.  12.  Mees  & Pledge,  “A  General  Dis- 
cussion,” British  Jour.  Sup.  to  vol.  57,  45  (1910). 

Supplies  and  directions  can  be  obtained  from  Herbert  & 
Huesgen  Co.,  456  Fourth  Avenue,  New  York. 


COLOR  PHOTOGRAPHY  187 

four  of  which  are  still  on  the  market.  The  theory 
is  the  same  for  all,  and  the  general  process  of  using 
the  plates  is  also  very  similar.  The  Autochrome 
was  the  first  commercial  success  and  is  still  probably 
the  best  though  some  of  the  others  are  better  in 
some  particulars.  The  theory  will  be  most  readily 
understood  from  a detailed  description  of  one  plate, 
the  Autochrome,  and  the  others  can  be  then  related 
to  it. 

120.  Autochrome.^^ — The  process  of  making  a 
plate  is  about  as  follows:  a quantity  of  wheat 
starch  is  sifted  to  separate  all  but  one  size,  about 
0.015  mm.  diameter.  The  sifted  starch  is  divided 
into  three  portions,  and  each  portion  is  dyed  one 
of  the  three  primary  colors.  The  three  portions 
are  mixed  very  thoroughly,  and  if  the  dyeing  has 
been  satisfactory  the  mixture  will  be  gray,  almost 
black.  A glass  plate  is  coated  with  a sticky  varnish, 
the  starch  grains  dusted  onto  it,  and  then  shaken 
off,  so  coating  the  plate  with  one  layer  of  the  col- 
ored grains.  The  interstices  are  filled  with  an 
opaque  material,  probably  by  dusting  a much  finer 
powder  on,  and  all  rolled  flat  and  varnished  with 
a waterproof  varnish.  On  holding  the  plate  up  to 
the  light  it  appears  gray.  If  it  did  not  it  would 

“If  supplies  cannot  be  obtained  from  a local  dealer,  they 
may  be  obtained  from  Lumiere  Jougla  Co.,  75  Fifth  Ave.,  New 
York. 


PHOTOGRAPHY 


i88 

impose  its  color  on  the  color  of  the  finished  picture. 
The  grains  are  so  small  that  the  eye  can  not  dis- 
tinguish the  individuals,  and  the  resulting  efifect  on 
a point  of  the  retina  is  the  sum  of  the  effects  of  a 
group  of  the  grains.  If  one  could  cover  up  part  of 
the  grains,  then  the  color  the  spot  would  appear, 
would  be  that  due  to  the  uncovered  grains.  The 
three  colors  used  for  the  grains  are  the  three  pri- 
mary colors,  red,  green,  and  blue  violet,  by  a suit- 
able mixture  of  which  any  color  may  be  imitated  to 
the  eye.  So  that  by  covering  up  a selected  set  of 
these  colored  grains  the  rest  will  imitate  any  color 
desired.  The  problem  is  to  cover  up  the  required 
set. 

The  next  and  final  step  in  the  preparation  of  the 
plate  is  to  coat  the  waterproof  varnish  with  a very 
thin  layer  of  panchromatic  emulsion.  These  plates 
are  packed  in  boxes  and  shipped  like  ordinary  dry 
plates. 

To  make  the  picture  the  plate  is  exposed  in  the 
camera  like  an  ordinary  plate  except  that  the  glass 
side  of  the  plate  is  put  toward  the  lens  and  allow- 
ance must  be  made  for  this  in  focusing,  either  by 
reversing  the  ground  glass,  or  by  moving  the  lens 
toward  the  plate  a distance  equal  to  the  thickness 
of  the  plate  after  focusing  on  the  ground  glass. 
Since  no  emulsion  can  be  made  equally  sensitive  to 
all  the  colors,  it  is  necessary  to  use  a color  screen 


COLOR  PHOTOGRAPHY  189 

to  lessen  the  effect  of  the  blue  and  violet  light.  Both 
the  mosaic  screen  and  the  color  screen  absorb  con- 
siderable light  so  that  the  exposures  have  to  be 
about  60  times  that  for  a medium  rapid  dry  plate. 
Also  since  the  emulsion  is  so  thin  the  latitude  of  ex- 
posure is  very  narrow,  and  exposures  have  to  be 
very  nearly  correct  to  be  at  all  satisfactory. 

The  exposed  plate  is  developed,  using  a definite 
time  or  following  the  progress  with  a green  light 
of  a particular  shade  for  which  the  emulsion  is 
not  very  sensitive.  The  plate  is  washed  and  im- 
mersed in  a solution  which  will  dissolve  the  finely 
divided  silver  forming  the  negative  image,  so  that 
there  will  be  clear  places  in  the  negative  everywhere 
where  there  was  negative  deposit,  and  negative  de- 
posit occurs  only  where  red  light  struck  the  red 
grain,  green  light  the  green  grain,  and  violet  light 
the  violet  grain.  So  that  when  the  deposit  is  re- 
moved these  grains  transmit  light  like  that  which 
came  up  to  them  in  the  camera.  It  is  only  necessary 
now  to  reduce  the  rest  of  the  silver  bromide  to 
metallic  silver  by  an  ordinary  developer,  in  order 
to  block  out  effectively  the  grains  which  did  not 
transmit  light  in  the  camera,  after  which  the  pic- 
ture stands  out  clearly.  Treatment  with  hypo  may 
be  omitted  when  all  of  the  undeveloped  silver  bro- 
mide has  been  reduced  by  the  second  development. 
After  the  film  is  dry  a coating  with  varnish  makes 


190 


PHOTOGRAPHY 


the  picture  more  brilliant  and  less  easily  damaged. 
To  protect  it  effectively  it  should  also  be  covered 
with  a piece  of  clear  glass. 

12 1.  Mosaic  Screen  Making. — It  will  be  evident 
from  the  above  that  the  central  point  of  the  method 
is  the  use  of  the  mosaic  screen,  and  the  different 
makes  of  plates  mentioned  above  differ  mainly  in 
the  character,  and  method  of  manufacture,  of  this 
screen.  The  first  such  screen  was  made  by  Joly, 
who  ruled  parallel  lines  in  the  three  colors  in  order 
to  cover  the  plate,  so  that  the  color  elements  were 
not  dots  but  extended  right  across  the  plate.  The 
MacDonough  screen  was  made  in  very  much  the 
same  way,  but  was  too  expensive  to  make,  and  it 
was  difficult  to  get  the  colored  strips  in  contact  and 
yet  not  overlapping. 

Another  promising  method  (Krayn)  was  to  make 
a pile  of  thin  sheets  of  celluloid  of  the  three  colors, 
cement  them  together,  and  then  take  thin  shavings 
off  the  edge  of  the  sheets.  This  would  give  the 
color  elements  as  lines,  but  they  could  be  made  into 
squares  by  cementing  a pile  of  these  shavings  to- 
gether so  as  to  mix  the  lines  and  shaving  again 
across  the  lines.  But  experiment  showed  unex- 
pected difficulty  in  making  thin  enough  celluloid 
sheets  and  also  in  making  the  shavings  thin  enough. 
The  elements  must  be  150-200  to  the  inch  to  be  at 


COLOR  PHOTOGRAPHY  191 

all  satisfactory,  while  the  specimen  the  writer  has 
seen  had  about  60. 

Many  of  the  other  screens  depend  on  the  dyeing 
of  a gelatine  layer,  for  example,  Warner-Powrie, 
Krayn,  Dufay,  etc.  In  making  the  Warner-Powrie 
screen  a layer  of  gelatine  or  fish-glue  is  sensitized 
with  chromate  and  then  exposed  under  a screen 
composed  of  alternate  opaque  and  transparent 
bands,  the  former  being  twice  the  width  of  the  lat- 
ter. The  soluble  part  of  the  gelatine  is  then  washed 
away  and  the  rest,  consisting  of  the  insoluble  bands 
immersed  in  a green  dye  which  is  afterwards  made 
fast  or  mordanted  by  immersion  in  tannic  acid  solu- 
tion. This  will  give  a plate  covered  with  green 
lines.  Another  layer  of  gelatine  is  then  spread  over 
these  green  lines  and  the  whole  process  repeated  but 
setting  the  screen  so  that  the  new  set  of  lines  (dyed 
red)  fall  between  but  next  to  the  blue  lines.  The 
plate  is  coated  a third  time  and  exposed  through  the 
other  lines  to  blue  light,  which  is,  therefore,  ab- 
sorbed by  the  green  and  red  lines,  so  that  the  gela- 
tine is  hardened  only  in  the  as  yet  uncolored  areas. 
The  plate  is  then  washed,  dyed  blue,  and  mordanted, 
the  process  seems  to  be  a good  one. 

Another  method  for  coloring  the  film  in  patches 
is  used  in  the  Dufay  screen.  A soft  gelatine  film 

“ Made  by  R.  Guilleminot  Boespflug  & Cie.,  22  Rue  de  Cha- 
teaudun,  Paris,  agent  in  U.  S.  being  George  Murphy,  57  E. 
Ninth  Street,  New  York. 


192 


PHOTOGRAPHY 


is  printed  in  a printing  press  with  parallel  lines  of 
a greasy  ink.  This  plate  is  then  immersed  in  a 
water  solution  of  a dye.  The  solution  will  not  pass 
into  the  gelatine  through  the  greasy  ink  but  only 
in  the  clear  places  between.  The  plate  can  now  be 
varnished  with  a waterproof  varnish.  When  dry 
the  application  of  a solvent  for  the  greasy  ink  and 
a little  friction  will  clear  off  the  greasy  ink  and 
its  varnish  covering,  leaving  the  clear  parts  of  the 
film  exposed,  but  the  dyed  strips  protected  by  the 
varnish.  The  process  is  then  repeated  but  with  the 
lines  running  across  the  previous  ones.  This  will 
give  two  colors,  the  first  lines  and  the  second  squares 
and  the  final  square  left  exposed  by  the  two  previ- 
ous processes  is  dyed  the  third  color.  The  Dufay 
screen  is  an  excellent  one  mechanically,  having 
sharply  marked  edges  to  the  color  dots  and  being 
reasonably  fine. 

The  latest  plate  on  the  market  is  the  Paget  Prize 
Plate,^^  in  which  the  mosaic  screen  and  sensitive 
plate  are  separate.  They  are  placed  in  contact  for 
exposing  but  are  separated  for  developing  and  mak- 
ing a contact  transparency,  which  is  bound  up  with 
a different  mosaic  screen.  This  allows  of  duplica- 
tion, but  the  screen  has  to  be  somewhat  coarse,  and 
there  is  the  serious  difficulty  of  securing  good  reg- 

May  be  obtained  from  Herbert  & Huesgen  Co.,  456  Fourth 
Ave.,  New  York. 


COLOR  PHOTOGRAPHY  193 

istration  all  over  the  plate  between  the  color  ele- 
ments and  the  positive. 

A great  many  other  methods  have  been  patented 
for  making  these  mosaic  screens.  A good  method 
offers  large  financial  returns. 

122.  The  use  of  these  mosaic  screen  plates  puts 
in  the  hands  of  the  careful  amateur  a means  of  tak- 
ing pictures  in  their  natural  colors  with  only  one 
exposure  and  with  comparatively  little  manipula- 
tion. The  pictures  are  satisfactory  in  many  ways. 
But  they  have  certain  disadvantages.  They  are  all 
transparencies,  and  copies  are  not  very  satisfactory, 
being  particularly  unsatisfactory  with  the  Auto- 
chromes. So  that  each  picture  requires  a new  ex- 
posure. The  most  satisfactory  method  of  reproduc- 
tion at  present  is  by  means  of  the  three  nega- 
tive-printing press  method,  where  the  mosaic  screen 
picture  is  treated  as  a colored  subject  to  be  copied. 
A method  usable  by  the  ordinary  worker  is  in  great 
demand.  The  following  method  offers  a good  deal 
of  promise. 

C1.ASS  3.  B1.EACH-0UT  Process 

123.  This  is  based  on  the  fact  long  known  that 
many  dyes  which  bleach  readily  in  the  light  are 
bleached  by  the  colors  of  light  which  they  absorb, 
and  which  is  the  complementary  color  to  which  they 
appear.  For  example,  if  a red  and  blue  dye  of  this 


194 


PHOTOGRAPHY 


type  be  mixed  and  coated  on  paper,  and  say,  blue 
light  be  allowed  to  fall  on  it,  the  red  dye  will  absorb 
the  blue  light  and  be  bleached,  while  the  blue  dye 
will  reflect  the  blue  light  and  remain  unbleached. 
The  paper  will  hence  turn  blue.  It  will,  therefore, 
only  be  necessary  to  mix  three  such  fugitive  dyes, 
of  the  three  primary  colors — red,  green,  and  blue- 
violet — coat  them  on  paper  and  expose  the  paper 
under  a mosaic  screen  picture.  Where  red  light 
gets  through  it  will  bleach  the  other  two  dyes,  leav- 
ing the  spot  red ; and  similarly  for  the  other  colors. 
The  mosaic  screen  picture  will  be  reproduced  in 
colors.  But  the  process  is  not  perfected  yet — one 
difficulty  being  to  make  the  colors  stop  bleaching 
when  the  printing  is  done,  that  is  to  ‘Tx'’  the  pic- 
ture. This  is  partly  attained  by  using  dyes  which 
bleach  much  more  rapidly  in  the  presence  of  certain 
reagents  like  anethol  for  example,  which  may  be 
washed  out.  The  other  serious  difficulty  is  that 
the  different  colors  of  light  bleach  the  dyes  at  very 
different  rates — so  that  for  instance  when  the  red 
is  bleached  the  blue  will  not  be.  Two  or  three  dif- 
ferent makes  of  such  papers  have  been  put  on  the 
market  but  they  do  not  seem  to  be  very  successful 
yet. 


CHAPTER  IX 


GOOD  PICTURES 

124.  A picture  is  a representation  of  a three-di- 
mensional subject  on  a two-dimensional  plane.  It 
is  a representation,  not  the  thing  itself,  and  as  such 
is  necessarily  a counterfeit  or  an  imitation.  Truth 
or  untruth  in  a picture  has  therefore  a very  special 
meaning,  in  fact  truth  is  determined  by  how  far  the 
picture  fills  the  purpose  of  the  worker.  A photo- 
graphic surveyor  desires  pictures  which  shall  be 
accurate  geometric  projections  of  the  subject  show- 
ing every  detail  near  and  far.  For  him  the  truth 
is  determined  by  how  far  his  pictures  fill  these  re- 
quirements. On  the  other  hand  a portrait  of  a 
familiar  person  has  to  represent  for  us  a variety  of 
changing  attitudes  and  expressions,  and  a detailed 
accurate  delineation  of  one  momentary  condition 
may  satisfy  this  desire  to  some  extent,  but  the  prob- 
abilities are  very  much  against  it  serving  as  well 
as  a picture  in  which  the  rendering  is  less  complete 
because  the  delineation  is  then  not  so  sharply  one 
mood  and  because  we  do  not  remember  the  fine 
detail  as  a rule  so  well  as  the  general  effect.  There 

195 


PHOTOGRAPHY 


196 

is  hardly  a professional  portrait  made  which  is  not 
retouched.  That  is  a portrait  is  to  be  judged  on 
the  basis  of  suggestion  of  the  person  pictured. 

125.  In  a general  way  pictures  may  be  divided 
into  two  classes: 

(a)  Record  Pictures,  where  the  aim  is  a geomet- 
ric rendering  of  the  subject.  Pictures  of  this  class 
are  of  the  very  greatest  importance,  particularly  in 
all  scientific  work.  The  purpose  makes  this  class 
include  the  great  majority  of  amateur  photographs 
taken  as  records  of  persons  and  places,  and  also 
the  great  majority  of  portraits.  There  is  no  doubt 
that  this  purpose  is  the  predominant  one  in  the 
vast  majority  of  pictures,  and  the  world  could  much 
better  afiford  to  lose  the  second  class  than  this  one. 

(b)  Pictorial  Pictures,  where  the  essential  pur- 
pose is  to  attract,  arouse,  and  generally  please  the  be- 
holder, not  so  much  by  the  particular  scene  pictured 
as  by  ideas  suggested.  As  such  they  appeal  to 
the  imagination,  and  to  attain  their  purpose  it  is 
not  at  all  necessary  that  the  object  forming  the 
actual  subject  should  be  suggested  by  the  finished 
picture.  In  this  class  are  to  be  placed  many  paint- 
ings and  a small  proportion  of  photographs.  Any 
liberties  with  the  geometry,  with  the  lighting,  with 
the  color,  are  justified  by  aiding  toward  the  object 
sought.  It  must  be  noted  however  that  such  lib- 
erties must  be  handled  in  a masterly  way  or  they 


GOOD  PICTURES 


197 


have  an  effect  very  different  from  that  intended. 

While  these  two  purposes  should  be  clearly  dis- 
tinguished and  acknowledged  in  their  extreme  ex- 
amples, there  are  relatively  few  pictures  which 
come  only  in  one  class.  The  first  ideal  of  the  young 
photographer,  as  also  of  the  early  school  of  photog- 
raphy, is  the  record  photograph,  and  so  well  under- 
stood is  this  ideal  and  so  useful  that  only  a few 
unbalanced  ''Art’’  workers  try  to  belittle  it.  But 
the  worker  soon  observes  that  while  equally  good 
records,  some  of  his  photographs  are  more  pleas- 
ing, are  turned  to  oftener,  and  are  exhibited  to  his 
friends.  He  begins  to  work  for  these  effects  as 
well  as  for  the  original  record  purpose,  and  he 
hence  begins  to  include  the  ideas  of  the  second 
class.  The  great  majority  of  workers  never  give 
up  precedence  of  their  first  purpose,  and  a study 
of  the  mass  of  pictures  of  those  who  have  makes 
it  seem  well  it  should  be  so.  Very  few  of  us  have 
the  temperament  or  time  to  be  artists,  but  we  can 
often  recognize  and  appreciate  good  imaginative 
work;  while  botched  work  of  this  kind  is  an  even 
more  serious  offense  to  the  ordinary  man  than  to 
the  artist  who  sympathizes  more  readily  with  its 
aim  and  who  understands  its  difficulty.  This  chap- 
ter is  written  for  the  ordinary  worker  who  is  pro- 
gressing sufficiently  with  the  straight  record  work 
that  he  desires  to  include  as  far  as  possible  more 


PHOTOGRAPHY 


198 

of  the  somewhat  intangible  features  which  make 
some  pictures  more  pleasing  than  others. 

126.  Picture  Composition. — The  choice  and  ar- 
rangement of  the  detail  forming  a picture  has  re- 
ceived the  general  name  of  composition.  The  sub- 
ject is  a very  extensive  one,  but  it  lacks  the  definite- 
ness and  precision  characteristic  of  the  sciences. 
Several  centuries  of  vigorous  controversy  by  both 
painter  and  critic  has  not  led  to  any  great  unanimity, 
and  any  generalizations  are  opposed  by  marked  ex- 
ceptions. Most  writers  admit  that  there  are  a group 
of  rules  followed  in  general  by  all  great  painters, 
though  at  times  they  may  break  any  of  them.  From 
the  discussion  it  also  appears  that  one  great  pur- 
pose served  by  such  controversies  has  been  the  atten- 
tion to  such  detail  thereby  promoted. 

The  literature  ^ upon  the  general  subject  is  vol- 
uminous and  varied,  but  the  great  majority  of  such 
books  are  by  painters  for  the  use  of  painters.  On 
account  of  the  great  diflference  in  method  be- 
tween painting  and  photography,  combined  with  the 
restrictions  inherent  in  photographic  composition 
and  the  loss  of  all  color,  these  books  are  not  very 
serviceable  to  the  young  photographer.  There  are, 
however,  an  increasing  number  of  books  ^ and  maga- 

^ Joshua  Reynolds,  Discourses;  Burnett,  Art  Essays. 

2 R.  H.  Poore,  Pictorial  Composition;  Sadakichi  Hartmann, 
Landscape  and  Figure  Composition;  H.  P.  Robinson,  Pictorial 
Effect  in  Photography. 


GOOD  PICTURES 


199 

zine  ® articles  written  especially  for  photographers 
which  well  repay  study. 

The  control  by  the  photographer  over  his  subject 
as  compared,  for  instance,  with  that  by  the  painter 
is  very  limited,  being  restricted  to  choice  of  subject 
as  a whole,  of  point  of  view,  and  of  lighting.  The 
treatment  following  exposure  may  also  modify  the 
picture  materially  but,  only  by  suppression,  seldom 
satisfactorily  by  addition.  In  the  following  articles 
an  attempt  is  made  to  review  systematically  only  the 
most  elementary  aspect  of  photographic  composi- 
tion, treating  it  in  the  way  it  is  apt  to  present  itself 
to  the  young  photographer  and  hoping  thereby  to 
awaken  an  interest  in  the  subject  that  will  lead  to 
further  study  elsewhere. 

127.  Unity. — Every  picture  needs  some  definite 
fairly  conspicuous  object  to  form  the  center  of  in- 
terest and  to  which  all  the  rest  of  the  picture  will 
be  subordinate.  It  is  the  reason  for  the  existence 
of  the  picture.  There  is  little  limitation  to  what 
may  be  so  used  except  that  it  must  have  enough 
human  interest  to  justify  the  important  position.  A 
human  figure  or  face  is  preeminently  such  an  object, 
and  its  treatment,  portraiture,  is  an  important 
branch  of  picture  making.  In  landscape  pictures  an 

® R.  H.  Poore,  “Figure  Composition,”  Photominiature , No. 
64;  F.  C.  Lambert,  “Pictorial  Principles,”  Photominiature,  No. 
53;  F.  Weston,  “Composition,”  American  Photo.,  p.  325  (1914). 


200 


PHOTOGRAPHY 


interesting  glimpse  of  a building,  or  a lake,  or  a 
pretty  shore  line,  or  a group  of  cattle,  may  readily 
serve  as  the  basis  of  a picture  and  justify  its  ex- 
istence. 

The  best  position  in  the  picture  for  the  center  of 
interest  varies,  but  no  one  advises  that  it  be  placed 
at  the  geometric  center.  Somewhere  near  but  not 
at  the  center  is  the  favorite  place.  When  placed 
very  far  from  the  center  it  requires  very  special 
arrangements  to  be  pleasing. 

128.  Simplicity. — To  be  satisfactory  a picture 
must  not  include  too  much  lest  it  divide  and  thereby 
weaken  the  interest.  The  commonest  mistake  with 
the  camera  is  to  seek  for  a point  of  view  which  will 
show  as  much  as  possible.  While  this  serves'a  rec- 
ord purpose  it' weakens  decidedly  the  appeal  as  a 
picture.  A compromise  between  the  two  purposes  is 
often  on  the  whole  a gain.  If  unnecessary  or  dis- 
tracting detail  cannot  be  omitted  by  choice  of  point 
of  view,  it  can  often  be  eliminated  later  or  at  least 
be  rendered  less  conspicuous  by  loss  of  its  fine  detail 
or  by  subdued  lighting.  To  be  effective  all  the 
subordinate  detail  should  be  naturally  associated 
with  the  main  subject  and  should  be  so  placed  if  pos- 
sible as  to  direct  attention  to  it.  For  the  landscape 
photographer  the  amount  of  such  choice  is  decidedly 
limited,  but  in  portrait  work  the  background  is  nor- 
mally entirely  controllable ; directly  around  the  head 


GOOD  PICTURES 


201 


it  is  preferably  of  diflferent  brightness  from  the 
head  outline  so  as  to  show  this  where  desired  and  to 
give  relief,  while  away  from  the  head,  detail  should 
merely  soften  the  blank  area  but  not  be  sufficient  to 
distract  attention  from  the  face. 

129.  In  landscape  work  the  foreground  offers 
special  difficulties.  On  account  of  the  nearness  to  the 
camera,  the  area  occupied  on  the  picture  is  relatively 
large,  and  thereby  small,  unimportant  detail  be- 
comes unfortunately  conspicuous.  Slight  sideways 
shift  of  the  camera  will  move  a conspicuous  bit  of 
detail  to  the  corner  or  partly  off  the  plate,  in  which 
position  it  may  possibly  help  the  composition,  or  it 
may  be  moved  off  the  plate  altogether.  Also  the 
foreground  may  occupy  too  large  a fraction  of  the 
area  of  the  print.  The  first  correction  usually  tried 
is  to  raise  the  lens,  but  if  feasible  it  is  usually  much 
better  to  raise  the  camera  itself.  Failing  these,  it 
becomes  necessary  to  trim,  the  print 

130.  The  position  in  the  picture  occupied  by  the 
sky-line  is  important.  Also  often  when  it  is  absent 
there  will  be  some  other  line  which  will  take  its  place. 
Our  habit  of  looking  horizontally  places  the  horizon 
somewhere  in  the  middle  of  the  view,  and  conse- 
quently the  observer  recognizes  the  view  point  most 
readily  when  the  sky-line  is  somewhere  near  the 
center  of  the  picture.  If  it  deviates  widely  from  that 
the  observer  has  to  recognize  a somewhat  unusual 


202 


PHOTOGRAPHY 


view  point — which  of  course  for  special  reasons  may 
be  a distinct  gain  in  case  the  rest  of  the  picture  en- 
forces it,  but  only  in  such  cases  is  it  an  advantage. 

131.  A print  where  the  sky  is  pure  white  paper  is 
at  a great  disadvantage  when  compared  with  one 
showing  clouds.  Cloud-forms  in  themselves  are 
often  very  beautiful,  and  they  have  such  significance 
for  us  in  our  daily  life,  that  a picture  using  them  to 
fill  in  an  otherwise  empty  area  gains  greatly.  There 
are  several  ways  in  which  cloud-forms  may  be  ob- 
tained in  the  print.  They  may  be  put  in,  using  a wad 
of  cotton,  or  a brush,  or  a pencil,  but  it  takes  great 
skill  to  make  them  look  natural  in  shading  and  form. 
Or  with  a little  care  they  may  be  printed  in  from 
one  negative  and  the  rest  of  the  picture  from  an- 
other. Outside  of  the  difficulty  of  matching  them  on 
the  sky-line,  is  the  greater  difficulty  of  making  the 
cloud  shapes  fit  the  horizon,  and  the  shadows  they 
cast  agree  with  those  in  the  landscape.  Without 
being  always  able  to  specify,  one  often  feels  that 
there  is  something  incongruous.  By  all  means  the 
best  procedure  is  to  take  both  clouds  and  landscape 
on  the  same  plate  at  the  same  time,  by  using  a suit- 
able color  sensitive  plate  and  a color  screen.  The 
temptation  to  make  the  clouds  taJ  prominent  is 
strong,  and  one  should  never  forget  that  both  clouds 
and  sky  are  very  much  brighter  than  the  earth. 
This  usually  requires  a rather  transparent  color 


GOOD  PICTURES 


203 


screen,  factor  of  two  or  four,  or  possibly  eight,  and 
the  whole  proceeding  requires  patient  watching  for 
a day  when  clouds  and  light  are  suitable. 

132.  Aerial  Perspective.— The  geometric  per- 
spective will  be  considered  later,  but  in  landscape 
work  there  is  an  effect  due  to  the  varying  density 
of  the  air — a great  exaggeration  of  which  one  gets 
by  looking  at  something  across  a hot  stove — leading 
to  slight  movements  of  the  image  and  therefore  blur- 
ring, whose  amount  depends  on  the  distance.  There 
is  sometimes  an  actually  visible  tremor  which  like  all 
motion  can  only  be  imperfectly  suggested  in  a pic- 
ture. Added  to  this  is  the  effect  of  dust  always 
present  in  the  air  but  in  greatly  varying  quantity. 
Its  effect  is  to  scatter  the  light  coming  from  a dis- 
tant object,  thereby  reducing  its  brilliancy  but  at  the 
same  time  to  add  to  its  brilliancy  by  light  scattered 
from  other  images.  The  result  is  a tendency  to 
bring  all  distant  objects  toward  the  same  brightness 
by  dulling  the  bright  ones  and  brightening  the  dark 
ones.  Also  since  the  added  light  is  a bluish  white, 
the  result  is  to  dull  all  bright  colors  by  an  admixture 
of  white  light.  Unfortunately  for  the  photographer 
the  amount  of  the  scattering  by  the  atmospheric 
dust  depends  on  the  color  of  the  light  being  mark- 
edly greater  for  the  blue  end  of  the  spectrum.  For 
not  very  luminous  objects  the  loss  of  blue  is  more 
than  made  up  by  the  added  light  which  has  been 


204 


PHOTOGRAPHY 


scattered  from  other  images,  the  result  being  the 
overlaying  of  all  distant  images  with  a marked  blue 
haze,  conversely  a reddening  of  all  relatively  bright 
objects,  as  in  sunset  and  sunrise.  The  photograph 
with  the  ordinary  emulsion  is  practically  taken  with 
the  blue  and  violet  light,  so  that  this  blue  haze,  be- 
sides leveling  light  contrast,  produces  so  much  de- 
posit in  the  negative  as  to  obscure  in  the  print  dis- 
tant detail  ordinarily  visible.  Hence  practically  dis- 
tant objects  require  less  exposure  than  near  ones. 
The  whole  objectionable  effect  in  the  plate  can  be 
corrected  by  using  a plate  whose  sensitiveness  curve 
is  approximately  that  of  the  visibility  curve  for 
the  eye,  see  Art.  51.  Or  the  same  result  may  be 
obtained  by  using  a color  sensitive  plate  and  a 
matched  color  screen  whose  combined  effect  is  to 
match  the  visibility  curve  of  the  eye.  With  suitable 
screens  the  effect  on  the  eye  can  be  imitated  as  ex- 
actly as  desired  or  the  rendering  of  distant  detail 
can  be  accentuated  or  obscured.  Except  for  special 
reasons  it  is  best  to  imitate  the  amount  of  detail 
the  eye  sees.  This  will  usually  mean  using  a good 
panchromatic  plate  and  an  adjusted  screen  and  giv- 
ing a time  exposure  with  a relatively  small  stop. 
The  time  exposure  also  allows  the  atmos- 
pheric tremor  to  produce  its  visible  amount  of  blur- 
ring. 

133.  Color  in  Subject. — The  necessity  of  trans- 


GOOD  PICTURES 


205 


lating  the  color  of  the  ordinary  subject  into  mono- 
tone leads  to  two  separate  difficulties,  one  purely 
photographic,  the  other  pictorial.  The  first,  that  of 
making  the  dry  plate  translate  the  color  according 
to  its  visual  brightness,  has  been  discussed  previous- 
ly in  some  detail.  Art.  51.  It  is  only  necessary  to 
add  here  a few  words  on  the  importance  of  doing  it 
correctly  by  the  use  of  good  color  sensitive  plates 
and  matched  screens.  The  more  pronounced  the 
color  the  more  obvious  the  improvement ; the  browns 
and  particularly  the  greens  in  landscape,  and 
freckles,  skin-tone,  and  blue  eye  in  portraiture,  are 
very  untrue  with  the  ordinary  plate,  and  repay  gen- 
erously in  truthful  rendering  and  personal  satisfac- 
tion any  added  difficulty  in  the  use  of  special  plates 
and  screens.  In  fact  one  can  hardly  urge  too 
strongly  their  uniform  use  for  all  subjects  showing 
definite  color,  distance,  or  clouds. 

The  second  difficulty  referred  to  above,  the  pic- 
torial one,  is  that  one  must  estimate  the  effect  of  the 
picture,  minus  color,  from  the  beautifully  colored 
image  on  the  ground  glass.  Until  hardened  by  repe- 
tition one  is  always  disappointed  by  the  print,  and 
one  of  the  major  factors  in  producing  this  feeling 
is  the  loss  of  color.  Practically  one  can  get  some 
assistance  by  using  a screen,  yellow  for  example, 
transmitting  a moderately  narrow  band  of  the  spec- 
trum. This  will  remove  the  color  from  the  image 


2o6 


PHOTOGRAPHY 


but  will  not  give  the  correct  brightness  to  the  colored 
objects.  One  has  to  be  wary  of  brilliantly  colored 
subjects,  such  as  flower  beds,  whose  color  may  be 
their  predominant  attraction.  The  effort  necessary 
to  observe  form,  light  and  shade,  and  to  neglect 
color,  leads  directly  to  greater  command  over  the 
factors  themselves. 

134.  Lighting. — Next  in  importance  to  subject 
and  point  of  view  is  the  lighting.  It  should  also  be 
made  to  serve  the  purpose  of  the  picture  in  exhibit- 
ing the  center  of  interest.  The  position  of  the  sun, 
that  is  the  time  of  day,  by  the  spots  lighted  and  the 
shadows  cast,  has  an  immense  effect  on  the  resulting 
picture.  No  one  can  advise  what  lighting  is  best;  it 
has  to  be  examined  and  tried  in  each  case  by  the 
worker.  One  can  say  however  that  as  a rule  harsh 
lighting,  that  is  direct  sunlight,  with  its  deep  shad- 
ows, is  not  as  desirable  as  a softer  more  uniform, 
lighting,  where  the  shadows,  being  somewhat  light- 
ed, show  some  detail.  Of  course  there  are  times 
when  glaring  sunlight  exhibits  the  aspect  one  wishes 
to  show — a camel  on  the  Sahara  desert  for  example 
— but  these  subjects  are  exceptional.  A partly 
cloudy  or  even  a heavily  cloudy  day  is  in  general  to 
be  preferred,  provided  of  course  there  is  not  motion 
enough  to  interfere  with  the  required  time  of  ex- 
posure. On  the  other  hand  the  lighting  can  be  too 
uniform,  so  that  the  shadows  are  hardly  noticeable. 


GOOD  PICTURES 


207 


and  one  of  the  important  features  by  which  we  rec- 
ognize shape  and  perspective  will  be  lost.  This  is 
particularly  noticeable  in  portraiture,  where  the  de- 
sired roundness  and  relief — representation  of  three 
dimensions — is  sought  after  regularly  by  a stronger 
lighting  to  one  side  and  over  the  head.  The  other 
side  and  below  must  not  be  neglected  entirely  but  be 
lighted  sufficiently  to  show  some  detail.  Consider- 
able variety  is  obtainable  by  varying  the  relative 
strengths  of  these  lightings,  thereby  varying  the 
amount  of  detail  in  the  shadows  and  the  amount  of 
suggestion  of  the  third  dimension. 

135.  Notan  (Chiaroscuro,  or  mass  effect), — 
There  is  another  aspect  of  composition  to  be  con- 
sidered which  is  of  great  importance  in  painting  and 
engraving,  so  much  so  that  whole  schools  have  been 
founded  on  it.  This  is  the  view  which  looks  upon 
the  whole  picture  as  groupings  of  masses  of  light 
and  dark.  The  detail  in  each  mass  is  temporarily 
neglected  or  blurred  over,  and  the  resultant  group 
of  areas  studied  as  to  whether  they  form  a beautiful 
pattern.  This  pattern  forms  a more  or  less  satis- 
factory basis  for  many  pictures,  so  that  the  artist 
may  neglect  entirely  to  fill  in  detail,  or  only  a meager 
amount,  thus  making  this  pattern  the  whole  picture. 
One  does  not  need  to  go  this  far  to  make  use  of  the 
idea,  and  every  composition  should  be  considered  as 


2o8  photography 

to  whether  the  grouping  of  the  large  masses  is  pleas- 
ing.'* 

136.  Balance. — The  most  important  considera- 
tion as  to  whether  this  pattern  will  be  pleasing  or 
not  is  what  is  called  balance.  In  a general  way  this 
means  that  if  the  picture  be  divided  vertically  (most 
important)  or  horizontally,  approximately  in  half, 
the  interesting  features  will  be  disposed  about 
equally  as  regards  general  interest  or  attractiveness 
on  each  side  of  the  dividing  line.  Also  it  is  to  be 
noted  that  an  interesting  detail  gains  in  its  efifect  on 
the  balance  as  it  is  moved  away  from  the  center, 
and  that  objects  showing  little  detail  do  not  hold  the 
attention  for  long,  so  that  their  importance  in  the 
balance  can  therefore  be  adjusted  by  changing  the 
sharpness  of  focusing.  When  one  first  looks  at  a 
picture  the  impulse  is  to  run  the  eye  swiftly  over  the 
whole  area,  and  the  first  impression  is  then  strongly 
influenced  by  the  balance.  After  the  first  quick  sur- 
vey one  settles  down  to  examine  the  interesting  de- 
tail. The  importance  of  unity  and  balance  in  the 
final  judgment  is  very  great. 

137.  Focal  Length  of  Lens. — There  are  some 
points  about  the  camera  which  by  their  effect  on  the 
satisfaction  from  the  picture  need  some  further  con- 
sideration. If  the  picture  is  to  be  a representation 

^ See  Poore,  Pictorial  Composition,  for  an  excellent  discus- 
sion of  the  various  possible  groupings. 


GOOD  PICTURES 


209 


of  a natural  scene  it  should  resemble  the  view 
through  a window  frame  with  the  observer  using 
only  one  eye.  If  the  eye  is  kept  in  one  position — 
for  example  by  observing  through  a hole  in  a fixed 
screen — then  the  scene  could  be  copied  with  a pencil 
faithfully  on  the  glass.  On  account  of  the  actual 
construction  of  the  human  eye  the  glass  to  be  dis- 
tinguished at  all  must  be  five  or  six  inches  from  the 
eye,  and  to  be  seen  with  the  greatest  comfort  and 
clearness  it  has  to  be  ten  or  twelve  inches.  Anyone 
observing  a small  picture  instinctively  adjusts  it  to 
this  distance  of  distinct  vision.  If  the  focal  length 
of  the  lens  used  in  making  the  picture  is  so  short  that 
to  get  the  view  through  the  window  effect  the  ob- 
server has  to  put  the  print  four  or  five  inches  from 
his  eye,  then  when  put  at  ten  or  twelve  inches  it  will 
not  look  natural.  The  perspective  will  not  look 
right,  the  foreground  will  be  exaggerated  in  com- 
parison with  the  distance,  and  parallel  lines  will  not 
converge  as  they  do  in  the  window  drawing.  This 
becomes  very  marked  in  some  of  the  beautifully 
compact,  convenient  vest  pocket  cameras,  where  the 
focal  length  of  the  lens  runs  around  one  and  a half 
inches.  On  this  basis  alone  the  ideal  focal  length 
would  be  ten  to  twelve  inches,  but  the  variation  al- 
lowable before  the  ordinary  eye  observes  it  is  large, 
so  that  lenses  of  five  inches  focal  length  are  moder- 
ately satisfactory.  Difficulty  on  account  of  too  great 


210 


PHOTOGRAPHY 


focal  length  is  rarely  met  with,  as  such  focal  lengths 
are  too  large  for  convenient  use,  Fig.  28,  p.  146. 

138.  Distortion  from  Short  Focal  Length. — The 
actual  size  of  the  image  on  the  plane  of  the  picture 
is  approximately  inversely  proportional  to  the  object 
distance  from  the  lens.  That  is,  the  tip  of  the  nose 
being  nearer  to  the  lens  than  the  eyes  will  be  drawn 
to  a greater  scale.  So  also  if  the  hands  are  held 
in  front  they  will  be  proportionately  too  large.  This 
is  inevitably  the  case  even  in  the  window  frame 
drawing  described  above.  The  difficulty  is  that  a 
picture  in  which  the  perspective  including  distortion, 
would  be  correct  if  viewed  at,  say,  five  inches,  is  ac- 
tually viewed  at  ten  inches,  so  that  neither  perspec- 
tive nor  distortion  match  with  one’s  experience,  that 
is  with  the  window  drawing.  If,  however,  the  sub- 
ject is  kept  at  a greater  distance  by  the  use  of  a 
longer  focus  lens,  the  perspective  and  distortion  be- 
come more  nearly  that  with  which  we  are  familiar, 
that  is  it  approaches  more  nearly  to  the  window 
frame  drawing,  see  Fig.  28,  page  146.  This  effect 
is  particularly  troublesome  where  in  portraits  and  in 
pictures  of  small  objects,  the  camera  is  moved  close 
up  so  that  the  area  occupied  by  the  image  will  be 
large  enough  to  show  the  desired  detail.  The  only 
solution  is  a longer  focus  lens. 

139.  Angle  of  View. — Using  both  eyes,  one  can 
readily  distinguish  moving  objects  where  the  angle 


GOOD  PICTURES 


2II 


of  view  between  them  is  i8o°,  which  is  greater  than 
any  photographic  lens.  But  the  detail  which  the 
eyes  can  observe  when  so  used  is  very  small  and  the 
effort  required  way  beyond  the  ordinary  use.  The 
user  of  spectacles  is  hardly  troubled  by  the  limita- 
tion of  his  angle  of  view.  It  seems  that  the  angle 
of  view  of  the  eyes  used  freely  and  without  direct 
effort  is  hardly  6o°.  In  the  camera  an  angle  of  6o  to 
70°  is  to  be  preferred,  and  one  can  make  a practice 
of  trimming  the  print  freely.  Angles  of  view  of 
more  than  70°  become  progressively  more  unpleas- 
ant because  they  demand  an  unusual  effort  on  the 
part  of  the  observer  to  interpret  the  picture. 

140.  Focusing. — On  account  of  the  narrow 
depth  of  focus  of  all  lenses  used  with  the  ordinary 
apertures,  it  becomes  necessary  to  select  a plane  in 
the  three-dimensional  subject  where  the  focusing 
shall  be  sharpest.  On  each  side  of  this  plane  the 
definition  will  fall  off,  the  most  rapidly  with  the 
largest  apertures.  Ordinarily  the  center  of  interest 
in  the  picture  is  the  place  to  focus  most  sharply,  and 
where  light  and  motion  conditions  permit,  the  choice 
of  aperture,  and  therefore  the  depth  of  focus  can  be 
profitably  considered  in  relation  to  how  strongly  one 
wishes  to  concentrate  attention  on  the  center  of 
interest  by  causing  the  neighboring  objects  to  go 
rapidly  out  of  focus.  For  example  the  practice  in 
portraiture  is  to  use  large  apertures  and  to  focus 


212 


PHOTOGRAPHY 


sharply  on  the  eyes.  This  has  the  effect  of  throwing 
the  ears  somewhat  out  of  focus  and  the  background 
completely  so,  and  serves  to  concentrate  attention 
on  the  part  of  the  face  normally  observed  closely. 
One  can  imagine  without  trying  it  the  effect  of 
focusing  sharply  on  the  ears. 

In  cases  where  detail  is  a very  subordinate  part 
of  the  picture,  the  rendering  may  be  made  more  ef- 
fective by  sacrificing  more  or  less  of  the  fine  detail. 
There  are  many  ways  of  doing  this,  for  example 
deliberately  throwing  a fine  lens  out  of  focus,  using 
a poorly  corrected  lens,  or,  better,  a lens  having  ad- 
justable spherical  aberration,  printing  with  a slight 
distance  between  negative  and  paper,  or  developing 
to  destroy  detail  in  a process  like  carbon  printing. 
Each  method  has  a somewhat  different  effect  in  the 
print,  and  the  method  used  should  be  chosen  with 
special  regard  to  its  particular  character.  A high 
class  modern  lens  gives  more  detail  than  is  effective 
in  the  average  picture ; it  gives  so  much  more  than 
the  eye  can  see  when  viewing  the  picture  as  a whole, 
that  the  observer  is  tempted  to  examine  parts,  and 
thereby  the  general  effect  is  weakened.  This  is  not 
to  say  that  every  picture  should  have  all  the  detail 
blotted  out,  an  effect  of  which  one  sees  far  too  much 
in  any  group  of  pictures  at  the  present  time,  as  if 
all  that  was  necessary  to  make  a picture  artistic  was 
to  blur  it.  All  such  devices  used  without  any  con- 


GOOD  PICTURES 


213 

sideration  of  their  appropriateness  serve  merely  to 
show  that  their  user  lacked  judgment. 

141.  Distortion  from  Inclined  Plate. — We  are 
so  accustomed  to  looking  horizontally  that  we  in- 
terpret every  picture  as  made  in  that  direction.  If 
one  takes  a picture  of  a tall  building  by  tilting  the 
camera  so  as  to  look  up  at  it,  and  then  holds  the 
print  vertically  for  observation,  the  building  will 
appear  to  be  falling  over.  But  if  the  print  be  tilted, 
upper  part  toward  the  observer,  in  agreement  with 
the  tilt  of  the  camera,  a position  can  easily  be  found 
where  the  representation  will  be  correct,  and  the 
building  look  vertical.  But  since  the  habit  of  look- 
ing perpendicularly  at  the  print  is  so  fixed,  a picture 
taken  for  any  other  viewing  is  misleading  or  escapes 
interpretation  unless  there  are  perfectly  obvious 
guides  to  the  required  orientation.  Also  this  re- 
quires effort  on  the  observer’s  part  which  is  not  ap- 
preciated, so  that  unless  physically  impossible  pic- 
tures should  be  made  with  the  plate  in  the  camera 
placed  carefully  vertical.  The  lens  may  be  tilted,  or 
shifted  up  or  down,  without  difficulty  in  this  respect. 

142.  Lens  Axis. — If  a picture  contains  a few 
straight  lines  it  is  easy  to  determine  fairly  closely 
where  the  axis  of  the  lens  intersected  the  plate. 
Even  if  there  are  no  straight  lines  the  effect  can  not 
be  entirely  ignored.  We  ordinarily  look  straight  at 
what  we  desire  to  see,  and  a picture  which  repre- 


214 


PHOTOGRAPHY 


sents  something  as  seen  above  or  below  while  look- 
ing straight  ahead  will  not  look  natural,  that  is  will 
require  special  effort  by  the  observer  to  interpret  it. 
Hence,  one  can  say  that  the  axis  of  the  lens  should 
fall  somewhere  near  the  central  part  of  the  picture. 
Pictures  where  the  lens  has  to  be  shifted  way  up  or 
down  to  get  the  subject  on  the  plate,  for  example 
tall  buildings,  are  not  the  most  pleasing  composi- 
tions. 

143.  Exposure  and  Development.  — ^The  expo- 
sure should  always  be  within  the  latitude  of  the 
plate  except  for  definite,  well-formulated  reasons. 
By  underexposure  of  various  degrees  it  is  possible 
to  subdue  the  shadow  detail  or  block  it  out  entirely, 
which  enables  one  to  imitate  moonlight  conditions 
and  to  emphasize  the  notan.  Actual  overexposure 
offers  hardly  any  results  which  can  not  be  better  ob- 
tained by  other  means.  The  ideal  development  is 
for  gamma  unity,  but  it  may  be  varied  from  (a) 
when  it  is  desired  to  over  or  under  exaggerate  the 
actual  contrast  in  the  subject,  or  (b)  when  the  full 
scale  of  the  printing  method  is  not  as  great  as  the 
range  of  illumination  in  the  subject  so  that  it  is 
preferred  to  lessen  contrast  rather  than  sacrifice 
detail  at  one  or  other  end  of  the  scale,  see  Art.  86. 

144.  Printing. — The  pictorial  element  is  impor- 
tant in  the  printing.  One’s  choice  of  the  various 
methods  is  based  on  many  considerations, — total 


GOOD  PICTURES 


215 


scale  needed,  rendering  of  detail,  tone,  surface,  ease 
and  certainty  of  control,  labor  required,  flexibility, 
permanency.  In  regard  to  tone  and  surface  there 
is  one  consideration  which  should  never  be  forgot- 
ten; anything  which  calls  attention  to  the  method 
and  away  from  the  picture  is  thereby  condemned. 
The  ideal  arrangement  is  where  these  details  har- 
monize so  well  with  the  picture  that  to  be  noticed 
they  must  be  looked  for  directly.  They  should  con- 
tribute to  the  pictorial  effect,  not  distract  attention 
from  the  thing  itself.  Very  brilliant  tones  are  for 
this  reason  almost  always  unsuitable,  as  also  are  ex- 
ceedingly coarse,  visible  grain  and  imitation  of 
other  fabrics. 

The  control  over  the  detail  rendered  and  the 
trimming  offer  however  the  greater  opportunity 
for  pictorial  improvement.  By  the  use  of  a glossy 
developing  paper  one  can  render  almost  all  the  de- 
tail in  the  negative,  and  by  successive  steps  to  a 
coarse  matt  the  fine  detail  may  be  more  and  more 
subdued.  This  control  however  extends  to  the  pic- 
ture as  a whole;  it  is  much  more  difficult  with  de- 
veloping and  printing  out  papers  to  control  locally. 
It  is  in  these  features  that  platinum,  carbon,  and 
gum  methods  have  the  advantage,  as  in  development 
one  can  make  marked  local  variations.  Also  with 
all  methods  local  shading  during  printing  enables 


2i6 


PHOTOGRAPHY 


one  to  control  definite  areas,  and  by  over-  or  under- 
exposing locally  reduce  the  detail. 

The  natural  desire  of  the  beginner  is  to  make  his 
print  cover  the  whole  negative ; and  it  is  only  when 
the  advantages  of  liberal  trimming  are  felt  that  this 
attitude  is  modified.  In  case  one  desires  a picture  of 
a certain  size  it  is  advantageous  to  enlarge  the  part 
of  the  negative  finally  selected.  To  determine  this 
part,  experiment  by  covering  the  four  edges  of  the 
print  with  four  pieces  of  paper  and  move  them  to 
various  positions  on  the  print  and  observe  the  effect 
on  the  picture  as  a whole.  Keep  in  mind  the  pre- 
dominant position  the  center  of  interest  should  oc- 
cupy, and  also  the  requirements  of  notan  and  bal- 
ance. A careful  study  of  one  print  you  are  inter- 
ested in  will  teach  a great  deal.  When  satisfied 
mark  the  print  and  trim  to  the  marks.  In  case  a 
white  margin  is  desired,  surround  the  selected  part 
of  the  negative  with  black  paper  pasted  to  the  gel- 
atine, and  print  through  this  mask. 

145.  In  mounting  try  to  make  the  mount  serve 
the  purpose  of  the  print  and  not  distract  from  it. 
When  several  pictures  are  mounted  together  on  one 
page,  choose  pictures  which  form  a harmonious 
group  by  having  some  common  features — similar 
printing  method,  similar  treatment,  similar  subjects. 
Choose  the  most  interesting  and  conspicuous  one  as 


GOOD  PICTURES 


217 

the  center  of  interest  and  group  the  others  around  it 
in  balance  and  to  help  the  central  interest. 

146.  Conclusion. — In  games  of  chance  and  skill 
like  whist,  bridge,  checkers,  chess,  or  even  billiards 
and  pool,  the  pleasure  to  be  derived  from  the  routine 
of  the  game — the  mechanical  arranging  and  holding 
of  the  cards  and  dropping  one  as  desired — is  largely 
gone  as  soon  as  it  is  mastered  reasonably  well.  For 
those  who  go  on,  the  interest  must  shift  to  the  intel- 
lectual problem  of  the  management, — order  of  play 
— of  the  complex  interrelated  group  of  factors, — 
the  cards, — and  the  checking  of  one’s  deductions 
and  inferences  by  the  results  achieved — tricks 
taken.  The  mechanical  part  becomes  relatively  very 
subordinate  except  as  it  retains  some  of  the  intel- 
lectual problem. 

So  also  in  picture  making,  the  pleasure  in  the 
mere  routine  of  exposing,  developing,  fixing,  print- 
ing, continues  only  so  long  as  it  presents  difficulties 
susceptible  of  mastery — that  is  essentially  intellec- 
tual. When  one  has  mastered  the  routine  suffi- 
ciently well  that  there  are  no  obvious  improvements 
possible,  then  the  work  will  be  laid  aside  to  be  called 
into  play  when  needed  for  other  reasons,  unless  new 
methods  are  experimented  with  or  unless  the  intel- 
lectual problem  of  the  building  of  the  picture  is  real- 
ized. This  new  problem  makes  use  of  the  mechani- 
cal routine  as  a means,  the  facility  v/ith  which  con- 


2i8 


PHOTOGRAPHY 


tributes  to  the  solution  of  the  new  problem.  For  this 
reason  the  pictorial  element  dominates  photo- 
graphic exhibitions,  relegating  methods  to  the  posi- 
tion of  means  to  this  end.  And  the  successful  ex- 
hibitor is  he  who  can  use  these  methods  freely  and 
skillfully  in  the  solution  of  this  more  complex  prob- 
lem. The  next  and  perhaps  final  step  in  the  series 
is  when  pictures  are  attempted  which  tell  a story  or 
represent  (and  thereby  transfer)  abstract  ideas  and 
emotions. 


APPENDIX 

Dkvelopmejnt 

Through  the  courtesy  of  Mr.  Alfred  Watkins  of 
the  Watkins  Meter  Co.,  Hereford,  England,  is  print- 
ed below  his  list  of  plate  speeds  and  development 
speeds.  The  numbers  apply  to  the  speed  of  the  plate, 
being  directly  proportional  to  the  speeds,  and  there- 
fore inversely  proportional  to  the  time  of  exposure. 
The  letters  apply  to  the  development  speeds,  thus 
placing  all  the  makes  of  plates  in  eight  classes.  The 
strength  of  developer  for  the  same  time  of  develop- 
ment is  indicated  by  the  drams  of  concentrated  de- 
veloper to  be  diluted  to  the  three  ounces.  The  same 
company  makes  the  concentrated  developer  ready 
for  use,  and  the  formula  is  given  in  the  Watkins 
Manual. 


319 


220 


PHOTOGRAPHY 


Watkins  Speed  List 

Compile  solely  from  actual  tests  for  use  with  Watkins 
Meters  (any  developer),  not  to  compare  speed  or  quality. 
Exposure  speeds  are  indicated  as  numbers  and  grouped ; thus 
i8o  means  a speed  somewhere  between  152  and  215. 

Development  speeds  are  indicated  by  code  letters,  the  fig- 
ures below  being  drams  of  two  concentrated  solutions  in  3 
ounces  of  complete  Watkins  Thermo  Pyro-Soda  Developer, 
for  6^  minutes  at  60  degrees.  This  applies  to  the  Time 
Thermometer. 

Where  two  brands  of  the  same  maker  are  quoted  alike 
they  may  differ  slightly,  but  samples  tested  came  in  the 
same  group. 

Development  Speeds 

WQ  VQ  Q MQ  M MS  S VS 
Drams i 2)4  3 4 $ 6^ 

Note  separate  heading  for  films  and  color  plates. 

Development  Speeds  have  nothing  to  do  with  Exposure 
Meters. 

The  words  Simplex,  Anti-Halo,  or  Backed,  do  not  indicate 
any  difference  in  speed. 


Agfa,  Ex.  Rap 

180  MQ 

“ Isolar 

130  MS 

Chromo 

250  Q 

“ Isolar 

130  MS 

‘‘  Isorapid 

250  MQ 

‘‘  Iso.  Rap. 

250  M 

Central  Ortho 

290  MQ 

Spec.  Non.  Hal. 

130  MS 

it  a 

180  M 

Comet 

130  M 

Cramer  Anchor 

130  MQ 

“ Banner 

180  S 

“ Crown 

250  s 

“ Isonon  Port. 

350  M 

“ “ Com. 

250  MQ 

APPENDIX 


Cramer  Polychrome 

180  MQ 

Spectrum 

180  MQ 

Trichrome 

180  MQ 

“ Iso.  Slow 

180  MQ 

‘‘  Med. 

180  MQ 

Inst. 

250  MQ 

Defender  Slow 

II  WQ 

Eastman  Rapid 

130  M 

Extra  Rapid 

180  M 

“ Ortho 

180  Q 

“ Spec.  Sens 

250  M 

Ultra  Rap. 

250  MS 

Forbes  Challenge 

90  VQ 

Hammer,  Record 

180  M 

“ Ex.  Fast 

250  MS 

“ Aurora  Ex.  Fast 

180  MS 

“ Ortho  Slow 

45  VQ 

“ Extra  Fast 

180  M 

‘‘  Non.  Hal. 

180  M 

Jougla  Intensive 

130  S 

Violet 

180 

“ Green 

130 

Seed,  23 

90  MQ 

26 

180  MS 

30  Gold  Edge 

350  MS 

L Ortho  Non.  Hal 

180  MQ 

Non.  Hal 

250  MQ 

Color  Value 

250  M 

Standard,  Thermic 

180  MQ 

“ Orthochrome 

180  MQ 

Stanley  Commercial 

180  MQ 

Roebuck  Blue 

180  S 

“ Ortho 

130  M 

Barnet  Ordinary 

4S  M 

“ Medium 

90  M 

‘‘  Ex.  Rap. 

250  S 

“ Studio 

250  S 

“ 550 

500  VS 

“ Press 

350  MS 

“ Spec.  Rap 

130  MS 

Super  Speed  Ortho 

250  S 

‘‘  S.  R.  Red  Diamond 

350  MS 

222 


PHOTOGRAPHY 


Bamet  Red  Seal 

250  S 

Med.  Ortho 

90  MQ 

“ Ex.  Rap.  Ortho. 

180  MS 

Self  Screen  Ortho 

250  MS 

Ilford  Ordinary 

45  Q 

“ Versatile  Rapid 

130  MS 

‘‘  Most  Rapid 

250  MS 

Ortho 

180  M 

“ Screened  Chromatic 

180  MQ 

‘‘  King’s  Own 

180  S 

“ Chromatic 

130  Q 

Empress 

90  MS 

“ Rap.  Chrom. 

180  M 

“ Spec.  Rap. 

130  VS 

‘‘  Monarch 

350  VS 

“ Zenith 

250  VS 

Panchromatic 

250  M 

Imperial  Ordinary 

45  Q 

“ Fine  Grain  Ord. 

16  WQ 

Sovereign 

130  MS 

Spec.  Sensitive 

250  S 

“ Spec.  Rapid 

180  MQ 

“ Flash  Light 

350  s 

Ortho  Spec.  Rap. 

130  MS 

Ortho  Spec.  Sens. 

180  MQ 

N.  F.  Ortho 

180  MQ 

Lumiere  Ex.  Rapid  (Blue  Label) 

90  MQ 

Ortho  A 

180  M 

Ortho  B 

130  MQ 

“ Ortho  C 

90  MS 

“ Sigma 

250  MS 

“ Ordinary 

6M 

Violet  Label 

500  S 

Rogers  Regular 

180  M 

“ Ortho 

180  MQ 

“ Non.  Hal. 

180  MQ 

Warwick,  Gold  Label 

180  M 

Silver  Label 

90  M 

Bronze  Label 

65  M 

“ R ainbow,  Fast 

350  M 

Slow 

180  Q 

‘‘  Warpress 

180  M 

APPENDIX 


223 


Wellington  Speedy  180  M 

Ex.  Speedy  350  MS 

Speedy  Iso.  180  M 

‘‘  Speedy  Portrait  180  M 

“ Landscape  65  Q 

Ortho.  Process  32 

Anti-Screen  180  MS 

‘‘  Spec.  Ex.  Speedy  350  MS 

“ Extra  Speedy  Press  250  MQ 

Xtreme  500  S 


Color  Screen  Plates 

Speed  when  used  with  maker’s  filter. 

For  Color  Plate  Meter,  also  See  Meter  with  bright  light 
and  large  stop  only. 


Autochrome  4 

Paget  1 1 

Dufay  5 

Omnicolor  6 


Films 


»U,”  ^Tack,”  ‘Autographic,”  and 

“ Kinematograph 

same  speed. 

Ansco  Extra  Fast 

180  S 

Austin  Edwards 

180  S 

Ensign 

180  MS 

Goerz  Tenax 

130  VS 

Kodak  N.  C. 

180  MS 

“ Ex.  Rapid 

250  S 

Lumiere  Plavik  (and  Block) 

130  MQ 

“ Cinemat 

130  M 

“ Sigma  Ortho 

180  S 

Premo  Film  Pack 

250  S 

“ “ “ Ex.  Rap. 

250  S 

Vulcan  Ortho 

180  S 

Pathe  Cine 

180  MS 

Criterion  Cine 

130  MQ 

Just  as  convenient  are  the  directions  issued  with 
the  ''Agfa'’  Rodinal  (Berlin  Aniline  Works,  213 


224 


PHOTOGRAPHY 


Water  St.,  New  York),  which  gives  detailed  direc- 
tions, list  of  development  times,  temperatures,  dilu- 
tions, and  list  of  plate  development  speeds;  all  by- 
Mr.  Watkins.  It  is  one  of  the  best  guides  available 
at  the  present  time  for  the  occasional  worker.  It 
all  applies,  of  course,  only  to  Rodinal,  which  is  an 
excellent  developer. 

Burroughs  Wellcome  & Co.  (35  W.  33d  St.,  New 
York)  publish  the  ‘Wellcome  Photographic  Expo- 
sure Record  and  Diary'’  yearly.  It  classifies  the 
plates  on  the  market  as  to  development  speeds,  and 
gives  also  development  times,  dilutions,  and  tem- 
perature coefficients  for  use  with  their  ready  pre- 
pared developers. 

The  Eastman  Kodak  Company’s  book  “How  to 
Make  Good  Pictures”  gives  full  directions  as  to 
time,  temperature  and  developer  for  use  with  their 
films.  Similar  information  is  given  by  the  Ilford 
Company  for  their  “King’s  Own  Plate,”  and  by 
Wratten  & Wainwright  for  some  of  their  plates. 


APPENDIX  225 


Table  of  Plate  Speed  Numbers 


Char- 
acter of 
plate 

H&D 

Watkins 

P 

Wynne 

FNo. 

Scheiner 

Degree 

n 

E 

6.80 

5 

7-36 

17 . 2 

c 

0.272 

5-67 

6 

8.82 

18.9 

b 

0.227 

Very 

4-85 

7 

10.3 

20.4 

a 

0.194 

slow 

3-78 

9 

13.2 

23.1 

I 

0.152 

2.84 

12 

17 . 6 

26. 1 

2 

0. 114 

2.27 

15 

22.1 

29.8 

3 

0.0905 

1.79 

19 

27.9 

33-6 

4 

0.0717 

1.42 

24 

35-3 

37-7 

5 

0.0567 

Slow 

1. 10 

31 

45-6 

42.9 

6 

0.0439 

0.871 

39 

57-4 

48.1 

7 

0.0348 

0.680 

50 

73-6 

54-4 

8 

0.0272 

0-531 

64 

94.1 

61.6 

9 

0.0213 

Ordi- 

0.415 

82 

120 

69.7 

10 

0.0167 

nary 

0.327 

104 

153 

78.5 

II 

0.0131 

0.255 

133 

196 

88.8 

12 

0.0102 

Rapid 

0.212 

160 

235 

97-4 

13 

0.00851 

0.157 

217 

319 

113 

14 

0.00627 

0.123 

276 

406 

128 

15 

0.00493 

0.097 

351 

516 

144 

16 

0.00388 

Extra 

0.076 

448 

659 

163 

17 

0.00303 

rapid 

0.060 

570 

838 

184 

18 

0.00239 

0.047 

727 

1070 

208 

19 

0.0018^ 

Where  E is  approximately  the  minimum  exposure  in  sec- 
onds for  an  open  landscape  at  noon  in  June  at  F 16.  And, 


Wynne  F number  = i35/\/i=  7.7VH  & D;  i=A(i.27)“ 
H&D=34/i;p=  50/i  =H  & D 50/34, 

where  *‘n”  is  the  Scheiner  degree  and  ‘A”  has  the  value  in 
this  case  of  about  4.4.^ 

^ See  also  J.  M.  Eder,  Handbuch,  vol.  i,  part  3,  page  240. 
Wratten  and  Wainwright,  Descriptive  list  of  dry  plates,  etc., 
page  31.  J.  M.  Eder,  Photo graphische  Correspondent,  37,  249 
{1900). 


PART  II 


LABORATORY  MANUAL 


PREFACE 


The  course  in  this  department  has  called  for  one 
lecture  and  three  laboratory  hours  per  week  over 
one  semester,  and  confers  two  credits.  No  student 
comes  to  the  laboratory  for  a smaller  interval  than 
two  hours.  The  work  has  been  offered  always  in 
the  second  semester  because  for  the  latter  half,  when 
the  student  has  had  some  experience,  the  weather, 
the  lighting,  and  the  foliage  are  good.  The  student 
opinion  has  been  that  the  work  required  fully  earned 
the  credits — which  feeling  is  to  be  desired,  as  the 
tendency  for  the  outsider  is  to  look  upon  such  a 
course  as  a ''play’’  course.  The  student  attitude 
toward  it  is  well  described  by  one  student’s  imme- 
diate objection  to  omitting  anything  from  the  list 
"Tut  increase  the  credits.” 

It  has  been  the  practice  here  to  furnish  all  the 
supplies  and  chemicals  as  well  as  apparatus,  and  to 
charge  a laboratory  fee.  There  has  been  no  diffi- 
culty about  pilfering  or  extravagance  since  a defi- 
nite statement  was  made  of  the  material  to  which 
the  fee  entitled  the  student,  excess  to  be  separately 
paid  for  and  all  supplies  to  be  delivered  only  on 
a signed  order.  This  plan  has  the  great  advantage 

229 


PHOTOGRAPHY 


230 

that  the  student  is  not  worried  about  the  cost  of 
each  experiment  and  hence  skimping  on  materials. 
The  laboratory  can  buy  to  better  advantage  than 
the  student,  and  uniformity  and  reliability  in  the 
supplies  can  be  assured. 

Physics  Laboratory, 

University  of  Wisconsin,  1917 • 


LABORATORY  MANUAL 


Generai,  Directions 

Each  student  is  provided  with  a small  locker  for 
his  own  special  material  and  apparatus.  When  as- 
signed this  locker  should  contain : 

I small  negative  drying  rack  2 pads  report  blanks 
I print  album  i binder  for  reports 

I empty  box,  4 x 5,  for  negatives  i box,  4x5,  dry  plates 

In  the  allotment  of  laboratory  places  the  student 
will  be  assigned  a partner  as  a rule.  It  may  not 
always  be  possible  that  the  student  shall  work  all 
his  time  with  one  and  the  same  partner.  The  two 
will  have  to  compare  work  done  and  take  up  what 
will  be  for  both  the  most  convenient  work  next  in 
order  in  the  manual.  On  account  of  the  time  re- 
quired for  the  drying  of  plates  and  prints  as  well 
as  long  continued  washings,  it  is  necessary  that  the 
student  carry  on  more  than  one  experiment  at  once, 
so  that  the  time  will  not  be  spent  in  idle  waiting. 
When  a stage  in  an  experiment  is  reached  requiring 
a long  wait,  look  back  to  see  that  printing  and 

mounting  is  up  to  date,  and  if  so  start  the  next  ex^ 

231 


232 


PHOTOGRAPHY 


periment.  Very  much  time  can  be  saved  by  intelli- 
gent planning  of  the  work. 

Beside  the  apparatus  in  the  individual  locker  each 
dark  room  should  have  on  its  shelves  at  the  begin- 
ning of  the  work  the  following: 


4 printing  frames 
I centigrade  thermometer 
I camel’s-hair  brush 
I print  washer 
I measuring  glass 

1 tube  paste 

2 hard  rubber  trays,  4x5 

2 hard  rubber  trays,  10  x 12 


1 print  stick 

2 wooden  tray  covers 
I negative  washer 

I glass  funnel 
I bottle  “soda^’ 

I bottle  ‘‘Pyro” 

I bottle  acid  ‘‘Hypo.’’ 


As  far  as  possible  each  article  is  marked  for  the 
locker  to  which  it  belongs — and  if  for  any  reason 
any  article  or  bottle  is  borrowed  from  another  room 
it  must  be  returned  where  it  belongs. 

When  the  experiments  require  apparatus  not 
listed  above  it  is  to  be  obtained  from  the  instructor 
and  returned  as  soon  as  out  of  use,  or  on  finishing 
the  day’s  work. 

At  the  close  of  the  period  all  apparatus  and  sup- 
plies are  to  be  returned  where  they  belong  and  the 
desk  left  clean  and  dry  for  the  next  group.  In  the 
case  of  spilled  solutions,  particularly  hypo,  it  must 
be  washed  carefully  into  the  sink.  The  sink  must 
not  be  made  a receptacle  for  waste  paper,  etc.,  as 
blocking  of  the  drain  may  flood  the  whole  labora- 
tory. The  work  for  the  period  will  have  to  be  so 


LABORATORY  MANUAL 


233 


arranged  as  to  allow  of  vacating  the  bench  promptly 
at  the  close  of  the  period  so  as  not  to  interfere  with 
the  succeeding  group. 

The  staff  reserves  the  right  to  impose  fines  for 
gross  carelessness  in  the  care  of  the  laboratory  and 
apparatus.  In  general,  small  routine  breakages 
are  covered  by  the  laboratory  fee,  but  in  case  of 
serious  breakage  or  injury  resulting  from  gross 
carelessness,  special  assessments  will  have  to  be 
made. 

As  each  experiment  is  finished  a report  covering 
the  following  items  is  to  be  handed  the  instructor. 

1.  Negative  and  print  data  sheet  filled  in. 

2.  Negatives  and  specimen  prints. 

3.  A statement  of  the  object  of  the  experiment  with 

brief  theoretical  discussion. 

4.  Statement  of  the  results  obtained  with  a criticism 

of  them. 

There  are  two  forms  of  report  blanks,  one  for 
routine  reports  on  negatives  and  prints,  the  other 
blank  for  reports  on  other  work.  Every  plate  should 
be  reported  on  whether  spoiled  or  not,  and  the  spe- 
cial directions  for  reports  on  the  particular  work 
followed.  In  every  case  with  the  report  is  to  be  sub- 
mitted the  material  (negatives,  prints,  etc.)  reported 
on.  These  reports  with  comments  will  be  returned 
if  possible  the  following  day,  and  the  first  item  of 
the  day’s  work  should  be  their  review  and  correc- 
tion. They  should  be  carefully  preserved  in  the 


234 


PHOTOGRAPHY 


binder  supplied  for  the  purpose,  and  at  the  end  of 
the  semester  form  the  laboratory  report,  which,  to- 
gether with  other  submitted  material,  determine  the 
laboratory  standing.  These  reports,  negatives,  al- 
bum, prints,  etc.,  become  the  property  of  the  stu- 
dent after  examination  at  the  close  of  the  semester 
when  the  laboratory  keys  are  returned. 

Expe)RIME:nt  I 

The  work  for  the  first  day  consists  in  making 
two  negatives.  Load  one  plate  holder  on  each  side, 
film  side  outward,  using  only  the  desk  ruby  lantern 
in  the  dark  room.  The  film  side  may  be  distin- 
guished by  the  feel- — the  finger  tips  slip  more  easily 
on  it  while  they  stick  more  to  the  glass  which  feels 
smoother.  Or  in  the  ruby  light  the  film  side  gives  a 
less  sharp  reflected  image  and  looks  brighter  gen- 
erally than  the  glass  side.  Avoid  as  much  as  pos- 
sible touching  the  film  with  the  fingers.  Take  the 
camera,  loaded  plate  holder,  tripod,  and  focusing 
cloth,  and  go  out  with  the  instructor  to  make  one 
exposure.  Return  to  the  dark  room  to  develop  the 
exposed  plate.  Mix  together  equal  quantities  (total 
2 or  3 oz.)  from  the  two  bottles  marked  ‘'Soda’’ 
and  “Pyro,”  using  a fresh  mixture  for  each  plate. 
Determine  the  temperature  of  the  mixture  with  the 
thermometer  and  refer  to  the  temperature-time  of 


LABORATORY  MANUAL 


235 


development  table  posted  by  the  sink,  for  the  time 
for  which  to  keep  the  plate  in  the  developer.  Im- 
merse the  plate  film  side  up,  remove  all  air  bells, 
cover  the  tray,  and  leave  it  there  this  predetermined 
time,  with  an  occasional  rocking. 

Rinse  under  the  tap  and  place  in  the  ^'hypo’’  solu- 
tion in  large  tray  till  all  the  white  in  the  film  disap- 
pears (5-15  min.).  Wash  in  changing  water  in  the 
washing  box  for  15  to  25  min.  Discard  the  used 
developer  but  return  the  hypo  solution  to  the  bottle. 

While  waiting  for  the  plate  to  ''fix’’  in  the  hypo, 
reload  the  plate  holder  and  repeat  the  above  process, 
making  a second  negative. 

When  washed  See  that  there  is  no  deposit  on  the 
film,  and  dry  the  plate  on  edge  in  the  rack.  On 
request  the  instructor  will  put  out  to  dry  any  plate 
not  finished  washing  at  the  end  of  the  period,  and 
the  drying  rack  (which  is  numbered)  should  be  left 
out  ready  to  receive  them. 

Submit  the  negatives  when  dry  with  the  report 
on  each. 

Expi;rimi:nt  2 

Repeat  Exp.  i making  negatives  two  at  a time. 
Do  not  take  more  than  two  exposures  on  one  trip 
out;  later  when  you  are  more  sure  of  the  routin-e 
and  will  not  waste  plates  uselessly  use  both  plate 
holders.  The  choice  of  subjects  to  photograph  is  in 


PHOTOGRAPHY 


236 

your  own  hands,  except  for  the  request  not  to  choose 
subjects  far  away  or  to  spend  too  much  time  over 
the  choice. 

Take  a look  at  the  light  outdoors  today  and,  con- 
sidering the  subject  to  be  photographed,  estimate  the 
exposure.  Consult  the  instructor  for  confirmation 
of  the  exposure  chosen. 

Expe)riment  3 

At  some  time  in  the  early  part  of  the  semester 
make  up  the  following  solutions  used  in  this  work. 
Bring  the  three  bottles  from  the  shelves  in  the  de- 
veloping room,  return  whatever  solution  they  con- 
tain to  the  stock  bottles,  and  make  up  sufficient  of 
each  solution  to  fill  the  corresponding  bottles.  The 
compositions  are  as  follows : 

Stock  Pyro  Stock  Soda 

Water  up  to  1000  cc  Water  up  to 1000  cc 

Pot.  Metabisulphite  1.4  g Sod.  carbonate  (dry)  40  g 

Pyrogallic  acid  8.0  g Sod.  sulphite  (dry)  50  g 

Dissolve  in  the  order  given. 

Attacked  by  the  oxygen  of  the  air 
so  keep  stoppered. 

Acid  H3HPO 

A B 

Water  2000  cc  Water  200  cc 

Hypo  500  g Sod.  Sulphite  (dry)  35  g 

Acetic  acid  (glac.)  30  cc 

Alum  (dry)  35  g 

Dissolve  A and  B separately  and  in  the  order  named, 
then  add  B to  A with  constant  stirring  and  add 
sufficient  water  to  make  2500  cc.  ^ 


LABORATORY  MANUAL  237 


Paper  Developer 


Water  up  to 
Metol 


1000  cc 


0.7s  g 

3-0  g 
12  g 
21  g 

0.4  g 


Hydrochinon 
Soda  Sulphite  (dry) 
Sod.  Carbonate  (dry) 
Pot.  Bromide 


Dissolve  in  the  order  named  in  some’what  less 
than  1000  cc  and  after  all  are  dissolved  add  v^ater 
enough  to  make  up  to  1000  cc.  Use  about  4 oz.  at 
a time  and  this  will  develop  3-4  doz.  prints;  never 
return  any  to  the  bottle  after  use.  It  does  not  spoil 
so  quickly  in  the  air  as  Pyro-Soda  does. 

Make  out  report  on  plain  report  blank  of  the 
actual  quantities  as  calculated  for  the  size  of  bottles 
used.  Did  you  observe  anything  which  will  be  of 
value  the  next  time  you  make  up  these  solutions  ? 


Prints  on  Developing  Paper  (Gas  Light  Paper). 
— Print  the  negatives  from  Exp.  i and  2 on  the  de- 
veloping paper.  Lay  the  negative  film  side  up  in 
the  printing  frame,  lay  a piece  of  the  paper  film  side 
down  on  the  film  side  of  the  negative,  fasten  the 
back  in  the  frame,  and  expose  the  glass  side  to  the 
light  either  at  the  room  door  or  before  an  incandes- 
cent light.  As  a first  trial  give  15  seconds  at  10 
inches  from  a 40  watt  tungsten  lamp.  Develop  in 
the  special  paper  developer  (Motol-Hydro)  as  the 


Expkrime;nt  4 


PHOTOGRAPHY 


238 

Pyro-Soda  stains  the  paper.  The  print  should  take 
one  to  three  minutes  to  develop.  If  it  is  dark  enough 
in  less  than  one  minute  the  exposure  has  been  too 
long.  In  making  a second  trial  change  the  expo- 
sure by  a factor  of  two  or  four.  The  print  will 
never  be  very  satisfactory  if  the  development  is  cut 
short  in  order  to  avoid  too  great  general  blackness. 
For  the  best  results  the  exposure  must  be  so  ad- 
justed that  on  a full  development  the  dark  parts 
will  be  black,  not  just  gray,  and  the  light  parts  will 
be  as  white  as  the  unexposed  margin.  If  the  unex- 
posed margin  becomes  gray,  the  development  has 
been  too  long,  or  the  developing  light  is  not  safe,  or 
the  paper  has  been  unintentionally  exposed  to  white 
light. 

If  with  an  exposure  just  short  enough  to  keep 
the  white  of  the  print  clear  the  dark  parts  do  not 
become  a strong  black,  then  the  negative  lacks  con- 
trast and  will  either  have  to  be  intensified  or  a more 
contrasty  grade  of  paper  used.  Conversely  if  with 
clear  whites  the  dark  parts  become  a dense  black  in 
which  all  the  detail  has  disappeared,  then  the  nega- 
tive is  too  contrasty  and  will  either  have  to  be  re- 
duced or  a less  contrasty  grade  of  paper  used. 
Refer  to  Text,  Art.  87. 

Rinse  the  developed  print  in  water  and  put  in  the 
hypo  for  15  minutes,  then  in  changing  water  in  the 
circular  washing  box  for  15-25  minutes.  Put  out  to 


LABORATORY  MANUAL 


239 


dry  face  up  on  a clean  paper  towel.  After  the  excess 
of  water  has  been  removed  either  by  drying  or  by 
the  towel,  the  print  may  be  turned  film  side  down 
and  will  then  dry  with  very  little  tendency  to  curl. 
Exercise  great  care  not  to  get  any  hypo  into  the 
print  developer  as,  if  the  slightest  trace  gets  in,  it 
is  liable  to  stain  all  the  prints  with  brown  patches 
which  may  not  show  till  the  prints  dry.  For  this 
reason  it  is  advisable  not  to  put  the  hands  in  the 
hypo  at  all  when  printing  but  to  handle  the  prints 
in  the  hypo  with  the  stick  provided.  The  paper  is 
not  nearly  as  sensitive  as  ordinary  plates  and  may  be 
handled  in  much  stronger  light;  the  hanging  red 
lamp  is  safe  only  for  short  exposures,  and  weak 
diffused  light  is  not  likely  to  bother.  But  if  the 
prints  at  any  time  get  gray  all  over,  the  light  may 
be  too  strong  or  the  development  too  prolonged. 

Fill  in  print  record  of  negatives  printed,  and 
hand  in  with  the  negatives  and  prints.  When 
returned  to  you  mount  in  your  album  a print 
from  each  of  the  negatives.  With  the  tip  of 
the  finger  rub  on  a narrow  band  of  paste  along  the 
upper  and  lower  edges  of  the  print,  and  fasten  in 
the  book.  Number  to  correspond  with  the  negative. 

Experiment  5 

To  Test  Systematically  for  Correct  Exposure 
make  a negative  exposed  in  strips  where  the  expo- 


240 


PHOTOGRAPHY 


sure  of  the  successive  strips  increases  by  a factor 
of  two.  In  order  to  make  the  exposure,  choose  the 
plate  holder  with  the  slide  marked  off  in  half  inches, 
draw  the  cover  slide  out  till  the  first  mark  shows 
and  give  the  exposure  No.  i in  the  table  (i.  e.,  i sec. 
at  stop  8).  Then  draw  out  to  the  second  mark  and 
give  the  second  exposure,  and  so  on  for  the  lo  ex- 
posures. Note  that  the  last  two  exposures  are 
alike.  The  first  strip  uncovered  will  get  all  the  suc- 
ceeding exposures,  the  second  strip  all  but  the  first 
exposure  and  so  on.  The  fourth  column  in  the  table 
gives  the  time  required  at  stop  i6  to  give  the  same 
exposure  as  that  actually  given  by  the  time  and  stop 
of  the  second  and  third  columns.  Then  if  the  whole 
group  of  exposures  given  each  strip  be  added  it 
will  give  the  fifth  column,  if  the  times  are  those 
required  for  stop  i6.  These  increase  very  nearly  by 
a factor  of  2.  The  changes  are  made  almost  entirely 
with  the  stop  as  the  shutter  speeds  are  very  unreli- 
able. 

For  an  ordinary  bright  day  the  series  of  expo- 
sures in  the  table  will  be  sure  to  have  some  in  the 
good-exposure  range. 

Choose  a subject  as  uniform  as  possible  across 
the  plate  as,  for  example,  the  uniform  front  of  a 
building  or  a uniform  landscape.  Consult  the  bul- 
letin board  for  suggestions.  Expose  one  plate,  de- 
velop and  fix  as  usual.  Note  the  change  in  char- 


LABORATORY  MANUAL 


241 

acter  of  the  various  strips,  and  make  your  choice  of 
the  best. 

Print  the  negative  making  good  prints  from  as 
many  of  the  strips  as  possible,  keeping  a record  of 


Table  10 


Strip  No. 

Exposure 

Equivalent 
Time,  Stop 
16 

Total 
Exposure 
of  Strip 

Time 

Stop 

I 

I 

8 

2 

4 

2 

I 

16 

I 

2 

3 

I 

32 

1/2 

I 

4 

I 

64 

1/4 

1/2 

5 

I 

128 

1/8 

1/4 

6 

l/2S 

8 

2/25 

1/6.25 

7 

1/25 

16 

1/25 

1/12.5 

8 

1/25 

32 

i/s° 

1/25 

9 

1/25 

64 

i/ioo 

1/50 

lO 

1/25 

64 

i/ioo 

i/ioo 

the  exposures  required  in  each  case.  What  is  your 
estimate  in  regard  to  this  plate's  latitude  of  expo- 
sure, that  is,  the  factor  between  the  least  and  great- 
est exposures  both  of  which  give  fair  prints? 

Experiment  6 

Factorial  Development. — Go  out  and  make  two 
good  exposures,  such  as  suggested  by  your  strip 
negative.  Flood  the  plate  with  the  usual  pyro-soda 
mixture,  noting  the  exact  time  with  the  second  hand 
of  your  watch.  Observe  the  plate  carefully  in  as 


242 


PHOTOGRAPHY 


good  a red-light  as  possible  for  the  first  appearance 
of  image  and  note  the  time  required  from  the  flood- 
ing of  the  plate.  Cover  the  tray.  Multiply  this 
time  required  for  the  first  appearance  of  image  by 
9 (see  text,  Art.  33c)  to  get  the  total  time  for  which 
the  plate  is  to  be  in  the  developer.  Rock  the  tray 
only  a few  times  during  development.  Finish  the 
plate  as  usual.  Repeat  the  process  with  other  plates. 
Submit  prints  and  negatives  with  report. 

Exp;^RiMr:NT  7 

Contact  Lantern  Slides.  — Go  out  with  the  camera 
and  make  two  exposures  suitable  for  lantern  slides. 
Take  a lantern  slide  matt  along  to  tell  what  size  to 
make  the  subject  on  the  plate.  Develop  by  factor  as 
in  Exp.  6.  When  dry  prixit  these  negatives  on  lan- 
tern slides  in  the  regular  printing  frame,  using  the 
exposure  time  for  the  first  trial  as  suggested  on  the 
bulletin  board. 

As  in  the  case  of  plates  and  paper  prints,  the  con- 
trol of  the  character  of  the  lantern  slide  is  best  un- 
derstood by  reference  to  the  discussion  in  Chap.  II, 
and  especially  Arts.  34,  83,  86  and  Exp.  4.  It  is 
even  more  important  than  in  prints  that  there  should 
be  no  silver  deposit  in  the  clear  places  (high  lights). 
If  the  slide  is  gray  all  over  the  exposure  has  been 
too  long,  and  if  at  the  same  time  the  dense  places  are 


LABORATORY  MANUAL 


243 


not  dense  enough  either  the  development  has  not 
been  long  enough  or  the  negative  lacks  contrast. 
Hence  in  general  it  is  best,  as  with  prints  on  paper, 
to  give  the  least  exposure  which  will  yield  the  de- 
sired maximum  density  if  the  development  is  pro- 
longed as  far  as  may  be  without  noticeable  fogging. 

Lantern  slide  plates  are  very  fine  grained  and 
much  slower  than  ordinary  plates.  A lantern  slide 
should  be  very  much  thinner  than  any  negative  to 
be  satisfactory;  when  the  picture  looks  clear  and 
strong  in  the  developing  tray  it  is  about  right,  long 
before  it  gets  black  all  over  as  a negative  does. 

On  account  of  the  small  amount  of  fog  produced, 
the  ferrous  oxalate  developer  is  an  excellent  one  for 
lantern  slides.  The  two  solutions  required  are  (a)  a 
saturated  solution  of  potassium  oxalate  and  (b)  a 
mixture  of  4 parts  of  water  and  one  part  of  a sat- 
urated solution  of  ferrous  sulphate  containing  about 
2%  sulphuric  acid.  The  latter  is  supplied  already  di- 
luted. Mix  one  ounce  of  each  and  use  for  developing 
only  one  slide,  as  it  oxidizes  rapidly  in  contact  with 
the  air  and  a precipitate  appears.  The  development 
starts  slowly  but  when  the  image  begins  to  appear  it 
comes  up  rapidly.  Since  there  is  no  need  to  fear  fog 
the  development  may  be  prolonged  for  15  minutes  or 
more.  Rinse  under  the  tap  and  soak  for  a few  min- 
utes in  a 1%  solution  of  oxalic  acid  to  remove  any 


244 


PHOTOGRAPHY 


iron  which  might  be  deposited  in  the  film.  Fix  as 
with  the  ordinary  plates,  wash  and  clean  carefully 
when  putting  out  to  dry.  Save  the  spoiled  lantern 
slides  so  that  they  may  be  cleaned  for  use  as  cover 
glasses.  Mount  the  slide  by  covering  the  edges  of 
the  film  with  a paper  matt  and  covering  this  with  a 
clean  piece  of  glass  of  the  same  size  as  the  lantern 
slide  and  binding  around  the  edges  with  strips  of 
gummed  paper.  Consult  the  instructor  for  hints  as 
to  the  easy  way  to  manage  this.  If  the  picture  re- 
quires a different  shaped  matt,  change  the  shape  of 
the  one  supplied  by  sticking  strips  of  gummed  paper 
- — the  gummed  binding  strip  will  usually  do — around 
the  sides. 

With  the  report  submit  the  negatives  used  as  well 
as  the  finished  slides.^ 

*The  saturated  ferrous  sulphate  solution  offers  certain  diffi- 
culties which  are,  however,  readily  surmounted.  In  the  pres- 
ence of  water  the  solid  ferrous  sulphate  hydrolyzes  somewhat 
as  it  goes  into  solution,  setting  free  sulphuric  acid  and  leaving 
undissolved  a basic  compound.  Hence  it  is  economical  but  not 
essential  to  add  a per  cent,  or  so  of  sulphuric  acid  to  the  water 
which  is  allowed  to  stand  on  the  crystals.  If  some  scraps  of 
iron,  such  as  iron  wire  or  nails,  are  added  this  iron  will  dis- 
solve slowly,  reducing  at  the  same  time  any  ferric  salt  present, 
and  in  this  way  red  effloresced  crystals  may  be  used  without 
any  hesitation,  but  the  solution  will  then  need  filtering.  This 
may  be  accomplished  without  tedious  waiting  by  inverting  the 
bottle  of  solution  over  the  funnel  with  the  neck  of  the  bottle  in 
the  funnel.  The  concentrated  solution  may  be  kept  for  years 
in  glass  stoppered  bottles,  if  the  stoppers  are  kept  well  greased 
and  the  solution  is  never  poured  from  the  bottle,  but  always 
removed  with  a pipette. 


LABORATORY  MANUAL  245 

Experiment  8 

Lantern  Slides  by  Reduction. — Set  up  the  con- 
centrated filament  tungsten  lamp  in  its  box  and  place 
the  condensing  lenses  and  water  cell  before  the  aper- 
ture. In  the  aperture  place  the  frame  with  the  4x5 
opening  for  carrying  your  negative.  See  that  there 
is  a ground  glass  between  the  negative  and  the  con- 
densing lens.  This  should  illuminate  the  negative 
uniformly. 

Put  the  4x5  back  in  the  view  camera,  and  set 
up  on  the  table  to  make  a picture,  lantern  slide  size, 
of  the  illuminated  negative.  Use  a kit  in  the  4x5 
plate  holder  to  carry  the  lantern  slide  plate.  Con- 
sult the  bulletin  board  for  the  first  trial  exposure. 
The  same  considerations  govern  the  exposure  as  in 
the  contact  slides. 

To  become  accustomed  to  the  developer  usually 
used  for  lantern  slides  develop  these  in  the  special 
hydrochinon  developer  in  which  contrast  grows 
fairly  rapidly,  while  it  does  not  stain  the  film  nor 
give  much  fog.  The  best  slides  are  usually  ob- 
tained by  exposures  requiring  2-4  min.  develop- 
ment. 

Hydrochinon  Developer 


A 

B 

Water  up  to 

1000  cc 

Water  up  to 

1000  cc 

Hydrochinon 

45  g 

Sod.  Carbonate,  dry 

30  g 

Sod.  Sulphite,  dry 

30  g 

Pot.  Carbonate 

90  g 

Sulphuric  Acid 

5 cc 

Pot.  Bromide 

8 g 

Sod.  Sulphite,  dry 

90  g 

PHOTOGRAPHY 


246 

For  use  mix  A and  B in  equal  quantities. 

Fix  as  with  the  ordinary  plates,  wash  and  clean 
carefully  when  putting  out  to  dry.  Mount  as  with 
the  contact  slides.  If  you  have  time  for  more  than 
your  own  negatives  exchange  negatives  with  some 
of  the  other  students. 

Report  any  difficulties  encountered.  How  do  you 
find  this  developer  compares  with  the  ferrous  oxa- 
late? Report  any  difference  whatever  which  you 
observe. 

Submit  the  finished  slides  with  the  negatives  used 
along  with  the  report. 

Expj^rimEnt  9 

Halation. — Take  three  negatives  of  the  same  sub- 
ject using  (a)  an  ordinary  plate,  (b)  ordinary  plate 
backed  with  black  paper.  Smear  over  a smooth 
4x5  piece  of  black  paper  with  sufficient  glycerine 
to  leave  the  surface  well  wet,  but  not  sufficient  to 
run  off  when  against  the  glass.  Lay  the  plate 
(glass  slide  down)  on  the  glycerined  paper.  Rub 
gently  into  contact  all  over,  being  very  careful  not 
to  get  glycerine  on  the  sensitive  film.  Load  as  usual 
and  expose  without  unnecessary  delay.  Wet  the 
plate  all  over  and  wash  off  the  glycerine  before  de- 
veloping. (c)  A double-coated  plate  whose  speed 
is  given  on  the  bulletin  board. 


LABORATORY  MANUAL 


247 


Choose  a subject  liable  to  show  bad  halation,  such 
as  a lighted  lamp  in  a room,  or  an  interior  with 
sunlight  coming  in  at  a window  or  the  fine  branches 
of  the  trees  against  a bright  background  of  sky  or 
lake,  in  general  wherever  very  great  sharply  marked 
differences  of  illumination  occur.  In  all  cases  the 
effect  is  more  obvious  in  the  print  than  in  the  nega- 
tive. The  test  is  made  particularly  severe  if  the 
development  is  prolonged  to  give  strong  contrast — 
conversely  for  avoiding  it.  Finish,  print  and 
report  as  usual,  paying  particular  attention  to  a 
comparison  of  the  results  with  the  different  ar- 
rangements a,  b and  c above. 


Experiment  10 

Intensification. — Make  two  underexposed  nega- 
tives by  exposing  a plate  or  Vie  that  judged  cor- 
rect. Lessen  the  exposure  by  manipulating  the  stop 
rather  than  the  time  on  account  of  the  unreliability 
of  the  higher  speeds  of  the  shutter.  Finish  as  usual, 
being  particularly  careful  to  wash  well  (i  hour). 
The  negative  should  be  very  thin  especially  in  the 
shadows. 

On  the  same  trip  out  expose  2 plates  as  correctly 
as  possible.  Better  choose  different  subjects  to  keep 
the  negatives  distinct.  In  the  development  cut  the 


248 


PHOTOGRAPHY 


time  to  about  of  the  usual  time  so  that  after 
fixing  it  will  be  very  thin.  Look  for  any  difference 
observable  in  the  character  of  these  negatives,  one 
pair  underexposed,  the  other  underdeveloped. 

When  they  are  dry  make  prints  from  them.  Note 
any  difference  in  the  character  of  the  prints.  Such 
prints  are  usually  flat,  that  is,  lacking  in  contrast. 
Refer  to  the  discussion  in  Chap.  II  and  especially 
Art.  34;  also  Arts.  83,  86,  Exps.  4 and  7.  Con- 
trast grows  with  the  progress  of  development 
until  fog  intervenes.  To  keep  the  print  from  be- 
coming too  dark  all  over  lessen  the  time  of  ex- 
posure as  much  as  possible,  and  to  get  the  maximum 
contrast  leave  in  the  developer  till  all  development 
appears  to  have  stopped  but  not  till  the  unexposed 
edges  of  the  print  become  gray.  It  is  possible  by 
varying  the  method  of  manufacture  to  make  devel- 
oping papers  which  vary  greatly  in  their  ability  to 
render  contrast.  Most  makers  offer  several  grades 
from  hard  contrast  for  flat  negatives  to  very  soft 
for  hard  contrasty  negatives  and  for  portraits.  The 
best  grade  to  use  depends  on  the  character  of  the 
negative  and  of  the  subject.  An  intelligent  choice 
of  these  papers  can  hardly  be  made  till  the  capa- 
bilities of  one  grade  are  known  from  experience, 
and  for  that  reason  we  will  use  only  the  one  grade 
made  for  correctly  exposed  and  developed  nega- 
tives. 


LABORATORY  MANUAL 


249 

With  your  report  submit  your  negatives  and  the 
best  print  from  each. 

Proceed  to  intensify  ^ these  thin  well-washed  neg- 
atives by  immersing  them  first  in  a solution  of  mer- 
curic chloride  made  up  as  follows.  The  solution 
is  exceedingly  poisonous. 

Mercuric  chloride  130  g 

Hydrochloric  acid,  cone.  3 cc 

Water  1000  cc 

The  mercury  salt  dissolves  slowly  and  there  is 
present  more  than  sufficient  to  saturate  the  solution. 
Leave  the  residue  in  the  bottom  of  the  bottle  and 
use  the  clear  supernatant  liquid.  Keep  the  used 
solution  clean  and  after  use  return  it  to  the  bottle. 
Allow  the  plate  to  lie  in  this  solution  till  it  just 
shows  white  through  the  glass,  then  remove  it  to  the 
washing  box  and  wash  for  an  hour.  If  necessary 

the  negative  may  then  be  dried  for  later  finishing. 

At  times  the  acid  in  the  mercury  solution  causes  the 
gelatine  surface  to  break  up  into  a mass  of  lines  re- 
sembling the  cross  section  of  the  cellular  structure 
of  a plant.  The  alum  in  the  hypo  bath  helps  to 
avoid  this  reticulation  but  if  persistent  with  the 
brand  of  plates  used  the  acid  should  be  cut  down 
to  half  or  less  than  that  given.  The  mercury  chlo- 
ride reacts  with  the  metallic  silver  thus 

HgCl2+Ag  = HgCU-AgCl 

■ See  text,  Art.  64. 


250 


PHOTOGRAPHY 


Both  resulting  chlorides  are  white  and  insoluble,  so 
that  the  washing  removes  only  the  excess  of  the 
mercury  chloride  and  the  acid. 

Develop  these  chlorides  in  the  daylight  with  a 
developer  made  by  mixing  3 volumes  of  a saturated 
solution  of  potassium  oxalate  with  one  volume  of 
saturated  solution  of  ferrous  sulphate."*  Allow  the 
plate  to  remain  in  this  till  thoroughly  black,  then 
wash  it  moderately  (15-20  minutes)  and  put  up  to 
dry.  Before  drying  see  that  the  surface  of  the  film 
is  clean  by  rubbing  it  gently  with  the  tips  of  the 
fingers  while  flushing  with  water.  If  the  washing 
water  contains  calcium  there  is  apt  to  be  a deposit 
of  calcium  oxalate  on  the  surface  of  the  gelatine. 

A very  great  increase  of  density  may  be  obtained 
by  blackening  the  plate  in  a dilute  solution  of  sodium 
sulphide  ® instead  of  in  the  ferrous  oxalate.  Try 
one  plate.  The  increase  is  apt  to  be  larger  than  is 
useful  except  in  special  cases.  Also  the  process  can 
not  be  repeated,  while  when  blackened  in  ferrous 
oxalate  it  may  be  repeated  many  times,  each  time 
about  doubling  the  density. 

When  dry,  print  the  negatives  again  and  com- 
pare these  prints  with  the  previous  ones.  Make  out 
reports  by  adding  the  new  treatment  to  the  previous 
history  of  the  negative,  and  submit  prints  and  nega- 

* See  Exp.  7. 

® See  Exp.  19. 


LABORATORY  MANUAL  251 

tives  again.  Finally  mount  a set  of  prints  in  the 
album. 

Experiment  ii 

Reduction. — Go  out  with  the  camera  and  make 
two  distinct  overexposures,  about  4-8  times  esti- 
mated correct  exposure.  Finish  as  usual,  and  when 
dry  make  prints  from  them.  If  the  incandescent 
lamp  requires  too  long  an  exposure  go  out  into  the 
daylight. 

To  reduce  the  negatives  soak  them  for  a few 
minutes  in  acid  hypo  diluted  with  its  own  volume 
of  water.  Then  remove  the  negative  and  add  to  the 
solution  a few  cc  of  a 10%  solution  of  pot.  ferri- 
cyanide,  say  10  cc  to  100  cc  of  the  diluted  hypo, 
stir  and  immerse  the  negative  again.  Watch  for 
appearance  of  thinning.  If  none  shows  in  1-2  min- 
utes repeat  the  addition  of  the  ferricyanide.  The 
mixture  is  attacked  by  the  air  so  that  it  does  not 
stay  active  very  long. 

Proceed  the  same  way  till  nearly  thin  enough, 
when  the  negative  should  be  put  quickly  into  chang- 
ing wash  water,  where  the  reduction  will  proceed 
a little  farther  during  the  first  part  of  the  washing. 
Dry  as  usual.  Record  the  potassium  ferricyanide 
and  hypo  used  for  future  assistance. 

Practice  this  on  some  others  of  your  dense  nega- 
tives. Print  these  reduced  negatives  and  compare 


252 


PHOTOGRAPHY 


with  the  previous  prints.  Report  by  adding  the 
account  of  the  treatment  to  the  previous  history  of 
the  negative  and  submit  negatives  and  prints  with 
the  report.  Finally  mount  the  prints  in  your  album. 

Expi:rime:nt  12 

Color-Sensitiveness. — Make  three  negatives  of  a 
colored  subject — an  ordinary  colored  print — ^with 
the  view  camera  in  daylight.  Use  the  special  4x5 
back.  Make  the  negatives  on  (a)  an  ordinary  plate, 
(b)  an  isochromatic  plate,  and  (c)  an  isochromatic 
plate  with  a color  screen.  The  color  screen  is  to  be 
placed  in  front  of  the  lens  and  as  it  absorbs  the  most 
effective  part  of  the  light  the  time  of  exposure  with 
it  has  to  be  lengthened.  Consult  the  bulletin  board 
for  the  necessary  increase  and  also  for  the  relative 
speeds  of  the  ordinary  and  isochromatic  plates.  The 
latter  is  sensitive  to  the  red  light  of  the  dark  room 
and  is  better  handled  entirely  in  the  dark.  Keep 
a record  of  the  plate  used  by  placing  a marked 
piece  of  paper  beside  the  print  being  copied. 

Develop  the  negatives  all  together  in  the  tank  for 
20  minutes  using  the  pyro-soda  diluted  in  propor- 
tion as  the  time  of  development  has  been  increased 
from  the  usual  time.  Measure  the  tank  and  make 
up  a volume  of  solution  sufficient  to  completely  cover 
the  plates.  Beware  of  using  the  tap  water  for  the 


LABORATORY  MANUAL 


253 


dilution  as  it  may  be  very  cold;  the  diluting  had 
better  be  done  with  distilled  water  which  has  stood 
in  the  room,  and  the  temperature  of  the  diluted 
developer  should  be  near  21°  C.  During  develop- 
ment reverse  the  plates  three  or  four  times  to  pre- 
vent them  developing  more  at  one  end  than  at  the 
other.  Fix  in  the  tray  or  in  the  tank,  but  before 
returning  the  tank  to  the  instructor  wash  it  so  thor- 
oughly that  you  can  guarantee  that  the  next  user 
will  not  have  hypo  in  his  developer. 

Compare  the  prints  of  these  negatives  with  the 
original  subject  and  in  your  report  discuss  the  ef- 
fectiveness of  the  various  colors  in  recording  them- 
selves on  the  plates  under  the  three  different  cir- 
cumstances used.  Submit  negatives  and  prints 
with  your  reports. 

Expe:rimknt  13’ 

Spectrum  Photography  for  Plate  Color-Sensi- 
tiveness.-— As  light  source  use  the  Nernst  lamp  set 
up  in  its  box  with  filament  horizontal.  Remove  the 
water  cell  and  the  condensing  lenses  and  replace  the 
negative  carrying  board  by  the  board  with  slit,  plac- 
ing the  slit  horizontal.  With  the  long  focus  achro- 
matic lens  focus  an  image  of  the  slit  about  four 
feet  away  and  set  up  the  view  camera  (see  dia- 
gram) so  that  the  ground  glass  will  be  in  the  same 


^S4 


PHOTOGRAPHY 


vertical  plane  as  the  image.  Remove  the  lens  board 
and  raise  the  front  of  the  camera.  Interpose  the 
replica  grating*  with  its  rulings  horizontal  in  the 
path  of  the  light  and  in  such  a position  that  the 
lower  spectrum  of  the  first  order  will  enter  the  open 
front  of  the  camera  and  fall  on  the  ground  glass. 


Fig.  50,  Exp.  13.  Diagrammatic  arrangement  of  apparatus 
for  photographing  the  spectrum.  A is  a ribbon  filament  tungs- 
ten lamp  or  a Nernst  lamp;  S an  adjustable  slit  set  horizontally 
in  the  wall  of  the  light  tight  box  enclosing  A;  B a long  focus 
achromatic  lens;  C a replica  grating;  and  D the  sensitive  plate 
at  the  back  of  a view  camera  without  a lens. 

Adjust  the  slope  of  the  beam  of  light  and  the  posi- 
tion of  the  grating  so  that  the  spectrum  will  be  a 
convenient  length  and  all  of  it  fall  on  the  ground 
glass.  In  the  view  camera  use  the  4x5  back  with 
long  edge  vertical.  Just  in  front  of  the  ground 
glass  is  a piece  of  cardboard  made  to  shift  side- 
ways to  four  numbered  positions  and  having  a slit 
through  which  the  spectrum  strikes  the  plate  while 
at  the  same  time  the  rest  of  the  plate  is  protected 

°R.  J.  Wallace,  “Use  of  Replicas,”  Astrophysical  Journal,  24, 
un  (igo7). 


LABORATORY  MANUAL 


255 


from  stray  light.  Along  the  side  of  this  slit  is  a 
transparent  film  with  a series  of  numbers  which 
throw  a shadow  on  the  ground  glass  or  sensitive 
film.  Make  a careful  record  of  the  colors  at  the 
different  numbers  so  that  these  numbers  on  your 
photograph  will  enable  you  to  tell  the  colors  which 
produced  the  deposit,  and  make  this  information 
part  of  your  report. 

Make  a series  of  three  exposures  on  each  plate  by 
setting  the  cardboard  screen  successively  at  three  of 
the  four  positions  and  at  each  shift  of  the  screen 
move  the  camera  sideways  so  that  the  spectrum 
will  continue  to  fall  on  the  opening  in  the  cardboard 
screen.  Make  each  exposure  four  times  the  pre- 
ceding one  and  consult  the  bulletin  board  for  the 
lowest  exposure  for  each  plate.  Do  this  for  three 
plates,  (a)  an  ordinary,  (b)  an  isochromatic,  and 
(c)  a polychromatic  plate.  To  keep  track  of  the 
different  plates  used,  mark  them  in  the  corner  with 
a steel  point  and  the  pressure  mark  will  develop  so 
as  to  be  visible  in  the  negative.  The  two  latter 
plates  are  sensitive  to  the  red  light  of  the  dark 
room  and  must  be  handled  entirely  in  the  dark. 
Develop  in  the  ordinary  way  in  the  pyro-soda. 

Make  a print  of  each  negative  on  a single  sheet  of 
paper  to  show  the  effect  of  longer  exposure  to  the 
different  colors.  Mark  the  colors  on  the  print  and 
the  exposures  on  the  different  strips.  Estimate  the 


PHOTOGRAPHY 


256 

different  densities  along  the  strip  and  draw  a curve 
showing  the  relation  of  color  to  density. 

Submit  negatives,  prints  and  curves,  with  your 
reports. 


Expe;rime:nt  14 

Dyeing  and  Testing  of  Plates. — The  stock  dye 
solution  is  made  up  by  dissolving  o.i  g of  the  dye 
(pinacyanol)  in  100  cc  of  alcohol  and  this  solution 
keeps  well  if  kept  in  the  dark.  For  use  15  cc  of 
this  solution  is  made  up  to  a liter  with  distilled 
water.  This  diluted  solution  is  on  the  laboratory 
shelves.  Soak  two  of  the  ordinary  unexposed 
plates  in  it  for  2.5  minutes,  shake  off  all  the  loose 
water  possible,  and  place  the  plates  on  edge  on 
the  rack  in  the  drying  cabinet  till  they  are  dry. 
Leave  an  empty  plate  box  numbered  with  your 
laboratory  number  beside  the  cabinet  to  receive 
your  plates  when  they  are  dry.  As  soon  as  the 
plates  are  bathed  avoid  all  light  as  much  as  possible 
as  the  dyeing  makes  the  plates  sensitive  to  the  red 
light  of  the  dark  room.  When  dry  test  these  plates 
for  color  sensitiveness  with  the  same  arrangement 
used  in  Exp.  12.  To  test  the  color  screen  at 
the  same  time  make  two  exposures  without  the 
color  screen  in  the  path  of  the  incident  light  and 
two  with.  Make  the  second  exposure  of  each  pair 


LABORATORY  MANUAL  257 

of  exposures  four  times  as  long  as  the  first,  and  for 
the  first  exposures  consult  the  bulletin  board. 

As  something  may  possibly  have  happened  to 
make  your  dyed  plates  fail,  make  two  exposures 
using  the  color  screen  on  a polychromatic  plate. 

Develop,  fix,  and  print  each  plate  as  usual.  As 
in  Exp.  12  draw  curves  representing  the  color  sen- 
sitiveness of  the  plate — one  curve  for  the  ordinary 
light  and  the  other  for  the  light  as  modified  by  the 
color  screen. 

Submit  negatives,  prints,  and  curves,  with  your 
reports. 

Expe:rime:nt  15 

Microphotography. — In  the  discussion  on  lenses, 
in  the  text  it  is  pointed  out  that  the  relative  sizes  of 
object  and  image  depend  on  their  respective  dis- 
tances from  the  lens.  As  the  object  is  brought 
nearer  the  lens  the  image  moves  away  and  grows 
larger.  For  great  enlargements  this  latter  distance 
becomes  unmanageable  with  the  ordinary  lens  and 
so  a short  focus  lens  has  to  be  substituted.  The 
object  glass  of  a microscope  is  essentially  only  an 
excessively  short  focus  lens.  Remove  the  lens  and 
lens  board  from  the  view  camera  and  replace  with 
the  microscope  with  the  tube  set  horizontal.  Close 
the  opening  around  the  microscope  tube  with  a black 
cloth.  Set  the  tungsten  lamp  and  condenser  so  as  to 


PHOTOGRAPHY 


258 

illuminate  the  object  brightly  in  line  with  the  tube 
and  focus  the  image  on  the  ground  glass.  This  may 
be  done  with  the  eyepiece  either  in  or  out,  with 
somewhat  greater  magnification  in  the  former  case. 

Good  illumination  depends  very  greatly  on  good 
adjustment  of  the  light.  The  focusing  may  be  done 
either  by  the  rack  and  pinion  of  the  microscope  or 
by  shifting  the  ground  glass.  Very  few  microscopes 
will  give  a picture  3^  X4,  all  of  which  will  be  in 
good  focus  at  once;  either  the  center  or  edges  will 
be  indistinct  (curved  image).  Also  the  visual  focus 
will  not  usually  be  the  photographic  focus.  The 
photographic  focal  length  will  usually  be  shorter 
than  the  visual,  so  one  errs  by  setting  the  ground 
glass  a little  too  near  the  microscope. 

Use  a lantern  slide  plate  in  a kit  in  the  ordinary 
4x5  holder.  Consult  the  bulletin  board  for  details 
as  to  exposure.  Make  two  good  negatives  of  differ- 
ent subjects,  finish  and  print  as  usual. 

Submit  negatives  and  prints  with  report 

Experiment  16 

Blue  Print  Paper. — Put  your  name  on  the  back 
of  two  8x10  sheets  of  paper  to  be  obtained  from 

’■  The  adherence  of  both  the  sensitive  materials  and  the  im- 
age may  be  greatly  helped  by  the  coating  on  the  surface  of  the 
paper.  The  paper  is  floated  on  or  passed  through  a dilute  solu- 
tion of  boiled  starch  or  a dilute  gelatine  solution  and  dried. 


LABORATORY  MANUAL  259 

the  instructor.  Wash  the  cameFs-hair  brush  thor- 
oughly to  be  sure  that  it  is  free  from  all  remains  of 
developer  and  with  it  paint  the  solution  of  ferric 
ammonium  citrate  and  potassium  ferricyanide  over 
the  paper.  This  may  be  done  in  daylight  and  it  is 
advisable  to  spread  it  as  evenly  as  possible  taking 
particular  care  that  no  place  is  missed.  If  there  is 
plenty  everywhere  the  unevenness  will  disappear  in 
the  printing  and  washing.  After  it  is  surface  dry 
take  it  to  the  instructor  to  put  in  the  box  over  the 
calcium  chloride  till  dry.  The  composition  of  the 
solution  is  given  below. 

Print  in  the  sunlight  under  a good  negative. 
From  time  to  time  open  one  half  of  the  back  of  the 
printing  frame  and  examine  the  print.  The  image 
will  show  clearly  and  must  be  overprinted  as  it 
bleaches  decidedly  in  the  subsequent  treatment.  A 
thorough  washing  in  water  and  then  drying  com- 
pletes the  print.  If  the  highlights  are  blue  the  ex- 
posure has  been  too  long,  the  negative  is  too  thin, 
or  the  paper  is  defective.  Test  for  defective  paper 
by  washing  it  without  having  exposed  it  to  light 
when  it  should  wash  white.  Foggy  prints  can  be 
improved  by  a quick  bath  in  a very  dilute  ammonia 
solution  followed  by  a very  dilute  hydrochloric  acid 
solution. 

For  comparison  with  your  own  paper,  make  some 


26o  photography 

prints  on  the  paper  made  by  a regular  manufac- 
turer. 

A B 

Potassium  ferricyan-  Ferric  ammonium  ci- 

ide  15  g trate  19  g 

Water  140  cc  Water  140  cc 

Mix  equal  parts  before  use  and  filter  if  there  is 
any  deposit.  Discard  the  mixed  solution  left  over. 

Expe:rim]^nt  17 

Gelatino-Chloride  Paper,  Also  Called  Printing 
Out  Paper  (P.  O.  P.). — Pick  out  several  of  your 
good  negatives  and  expose  the  P.  O.  P.  glossy  sur- 
face under  them  in  direct  sunlight  for  3 to  30  min- 
utes, depending  on  the  negative  and  the  light.  The 
progress  of  the  printing  may  be  judged  by  lifting 
half  of  the  back  of  the  printing  frame  and  observ- 
ing the  paper.  The  printing  should  be  carried  dis- 
tinctly beyond  the  desired  final  density,  as  it 
bleaches  markedly  in  the  toning  and  fixing. 

Wash  the  print  for  about  10  minutes  in  changing 
water  or,  better,  till  the  water  leaving  the  print 
shows  no  cloudiness.  Then  tone  in  a gold  chloride 
bath  made  by  adding  90  cc  of  water  to  10  cc  of  a 
stock  gold  chloride  solution  from  the  laboratory 
shelf.  Test  this  bath  with  litmus  paper  and  make 
it  neutral  or  slightly  alkaline  by  adding  some  of 
the  saturated  solution  of  borax.  Immerse  the  print 


LABORATORY  MANUAL 


261 


in  this  bath  and  the  color  should  change  slowly 
from  shades  of  red  through  purple  to  black  in  from 
four  to  eight  minutes.  If  the  print  shows  no  change 
in  one  or  two  minutes  and  the  bath  is  neutral  or 
slightly  alkaline,  remove  the  print  and  add  a few  cc 
more  of  the  stock  gold  solution,  stir,  test  for  acidity, 
and  replace  the  print.  When  toned  to  the  desired 
color,  wash  in  water  for  a few  minutes,  and  fix  for 
10  minutes  in  a hypo  bath  whose  composition  is 
given  below.  The  ordinary  acid  hypo  will  usually 
ruin  the  print.  Finally  wash  for  about  an  hour 
in  changing  water,  keeping  the  prints  well  sepa- 
rated. Dry  face  upward  on  a towel  as  they  stick 
strongly  if  face  down.  These  prints  still  have  the 
reputation  of  yielding  the  best  and  finest  detail. 

Report  the  work  done  and  any  observations  which 
would  help  you  to  do  the  work  better  next  time. 
Prepare  at  least  four  prints  for  mounting  finally 
in  your  album  but  first  submit  them  for  examina- 
tion along  with  the  negatives  and  the  reports. 

Stock  Gold  Chloride  Hypo  Solution 

Gold  chloride  15  grains  Water  2000  cc 

Water  450  cc  Hypo  180  g 

Alum,  dry  60  g 

Sodium  sulphite,  dry  6 g 

and  when  all  dissolved  mix  with 


Borax 
Water,  hot 


25  g 

300  cc 


262 


PHOTOGRAPHY 


and  let  stand  over  night.  A heavy  precipitate  will 
form  and  settle*  Use  the  clear  supernatant  liquid. 

Experiment  i8 

Enlarging. — Set  up  the  lamp,  water  cell,  con- 
densing lenses,  and  negative  carrying  board  as  for 
reduction  lantern  slides  (Exp.  7).  Select  one  of 
your  best  negatives  and  mount  it  in  the  window  film 
side  away  from  the  light.  Set  the  open  back  of  the 
view  camera  up  against  the  negative  carrier  so  that 
the  only  light  getting  into  the  room  has  to  pass 
through  the  camera  lens. 

Open  up  the  stop  in  the  lens  and  focus  the  picture 
carefully  on  the  vertical  board.  You  will  find  by 
shifting  the  lens  vertically  and  horizontally  that 
there  is  one  position  for  best  illumination.  Adjust 
the  size  of  the  picture  to  suit  the  sheet  of  sensitive 
paper  by  shifting  this  board  backward  or  forward 
and  re-focusing.  Fasten  the  paper  on  the  board  by 
using  two  glass-headed  pins  each  on  bottom  and  on 
each  side,  and  slip  the  paper  in  behind  the  pins  by 
leaving  them  just  loose  enough.  Adjust  the  picture 
and  focus  it  on  a piece  of  white  paper.  If  a piece 
of  red  glass  be  placed  over  the  front  of  the  lens 
the  resulting  red  light  can  be  thrown  without  harm 
directly  on  the  sensitive  paper  which  will  aid  in 
adjusting  it  in  position.  When  all  adjusted,  close 


LABORATORY  MANUAL 


363 

the  shutter,  slip  the  sensitive  paper  into  position, 
set  the  diaphragm,  and  expose  with  the  shutter. 
The  exposure  depends  so  much  on  conditions  of 
lighting,  size  of  enlargement,  density  of  negative, 
and  speed  of  paper  that  it  is  a matter  of  guess  and 
try.  Consult  the  bulletin  board  for  a suggestion  as 
to  the  first  trial  exposure.  With  regard  to  contrast 
the  same  remarks  apply  in  this  case  as  in  the  case 
of  contact  prints  (Exp.  4)  and  lantern  slides  (Exp. 
7)-* 

It  will  be  economical  of  paper  if  you  make  the 
first  trials  on  ^ or  % sheets.  If  one  ground  glass 
does  not  make  the  lighting  uniform  put  a second  in 
contact  with,  but  ground  glass  away  from,  the  nega- 
tive. Note  that  this  paper,  usually  called  Bromide 
Paper,  is  very  sensitive,  the  emulsion  is  much  the 
same  as  that  used  on  plates,  and  hence  should  be 
exposed  only  to  the  safe  red  light.  Develop  in  the 
metol-hydro  developer  in  the  large  tray.  Dilute  this 
developer  with  at  least  an  equal  volume  of  water  to 
slow  development.  Immerse  the  paper  in  the  devel- 
oper by  sliding  it  in  face  down  over  the  edge  of  the 
tray,  quickly  and  steadily.  When  wet  all  over  the 
surface,  lift  out  and  lay  back  face  up,  and  keep 
developer  always  over  the  surface.  A rinse  before 
the  hypo-bath  helps  to  avoid  stain,  and  so  also  will 
laying  it  face  down  in  the  hypo  and  then  turning 

• See  also  Text,  Art.  87. 


PHOTOGRAPHY 


264 

over.  If  any  part  of  the  print  stands  up  out  of  the 
hypo  even  for  a few  minutes  it  is  very  liable  to  be 
stained.  This  paper  is  too  large  for  the  print 
washer  and  will  have  to  be  washed  by  changing  it  a 
sheet  at  a time  from  one  tray  to  another  with  con- 
stant change  of  water,  say  12-15  changes.  Dry  as 
usual  on  towels. 

Submit  the  negatives  used  and  the  enlargements 
with  your  reports.  State  any  trouble  encountered, 
the  best  exposures,  and  your  own  opinion  of  your 
results. 

Expdrimdnt  19 

Sulphide  Toning  (Sepia). — May  be  applied  to 
the  contact  prints  or  the  bromide  paper  enlarge- 
ments, but  the  bromide  paper  is  much  the  more 
liable  to  spot  and  blister.  Prints  which  have  been 
thoroughly  dried  once  since  making  are  less  liable 
to  spot.  The  prints  fade  somewhat  in  the  process 
and  hence  require  to  be  printed  a little  darker  than 
your  taste  calls  for.  If  you  have  no  spare  prints 
suitable,  make  some  but  let  them  dry  thoroughly 
before  proceeding.  It  is  advisable  in  all  cases  that 
the  hypo  should  contain  an  acid  hardening  solutipn, 
and  must  have  been  well  washed  out  of  the  print. 
Bleach  the  print  in  the  solution  (formula  below) 
till  only  a faint  image  remains.  Wash  till  all  the 
yellowness  from  the  ferricyanide  solution  has  gone. 


LABORATORY  MANUAL 


265 

Immerse  in  the  sodium  sulphide  solution  (formula 
below)  till  of  an  even  tint.  Wash  moderately.  Dry. 

Bleacher^  Sodium  Sulphide  Solution 

Potassium  Ferro-  Sodium  Sulphide  ^ 10  g 

cyanide  40  g Water  100  cc 

Potassium  Bromide  10  g 

Water  1000  cc 

Boil  the  sulphide  solution  to  precipitate  the  iron, 
and  filter.  Dilute  i-io  for  use.  That  supplied  is 
diluted  ready  for  use. 

Submit  toned  prints  with  report. 

ExpErime^nT  20 

WET  COEEODION  PEATES ® 

Preparing  the  Glass. — Lantern  slide  size.  Wash 
first  under  the  tap  to  remove  all  coarse  dirt,  then 
by  immersion  for  3 or  4 minutes  in  boiling  hot 
alkali  and  soap  solution;  wash  well  under  the  tap, 
handling  the  glass  entirely  by  the  edges,  and  rinse 
in  distilled  water.  Dry  thoroughly  by  warming 
over  a Bunsen  flame,  holding  the  plate  by  the  fingers 
on  the  edge  and  keep  the  plate  moving.  If  you 
have  time  to  wash  several  plates  and  let  them  dry 
by  themselves  on  the  rack  they  will  be  much  clearer. 
Forming  the  Film. — As  soon  as  the  glass  plate 
® See  Foxlee,  ‘Wet  Collodion  Process,”  British  Journal,  54, 

483  (1907). 


266 


PHOTOGRAPHY 


has  cooled  to  room  temperature,  use  the  brush  to 
run  a streak  of  the  rubber  solution  about  % inch 
wide  all  around  the  edge  of  the  plate  to  help  keep 
the  film  attached  to  the  glass  plate.  Then  hold  the 
plate  horizontal  by  one  corner  with  the  left  hand, 
and  pour  a generous  pool  of  iodized  collodion  near 
but  not  touching  the  thumb.  Spread  the  pool  by 
gentle  tipping  so  it  flows  along  the  far  side  of  the 
plate  and  then  toward  you  along  the  edge  opposite 
the  hand,  which  usually  completes  the  covering  of 
the  plate.  The  corners  are  not  important  but  avoid 
getting  any  collodion  on  the  under  side  of  the  plate 
or  on  the  holding  fingers.  To  make  a thick  film 
allow  the  collodion  to  rest  horizontally  on  the  plate 
for  1-2  minutes,  then  pour  off  the  excess  into  the 
bottle  again  from  the  near  right  hand  corner,  keep- 
ing the  plate  rocking  to  avoid  streaks.  The  plate 
must  be  uniformly  coated  and  free  from  dust  to  get 
good  results.  All  the  above  can  be  done  in  any 
light  but  the  following  must  be  done  in  the  hanging 
red  lamp  light,  avoiding  long  exposures. 

Sensitizing. — As  soon  as  the  film  has  dried 
enough  to  feel  firm  on  the  corners,  2-4  minutes,  it 
is  to  be  placed  on  the  silver  wire  frame  and  lowered 
gently  and  steadily  into  the  silver  nitrate  solution 
in  the  glass  box  inside  the  sloping  wooden  box.  If 
the  edge  of  the  solution  stops  at  any  place  during 
lowering,  it  will  leave  a mark.  Leave  in  the  silver 


LABORATORY  MANUAL  267 

bath  3-4  minutes.  On  removing  the  plate  handle  it 
by  the  edges  and  if  the  fingers  become  stained  by 
the  silver  nitrate  wash  thoroughly  in  water  and 
remove  the  stain  by  soaking  in  the  ferricyanide  hypo 
reducer.  If  any  of  the  developer  remains  on  the 
fingers  or  under  the  nails  they  will  be  stained  blue 
by  the  ferricyanide.  The  film  may  become  loose 
from  the  glass  (a)  if  the  glass  is  not  clean,  (b) 
if  the  glass  was  not  dry,  (c)  if  the  film  was  not  al- 
lowed to  dry  sufficiently  before  placing  in  the  silver 
solution.  On  the  other  hand,  if  allowed  to  dry  too 
much  the  negative  is  apt  to  be  thin. 

Exposing. — Do  the  exposing  in  the  special  plate 
holder  which  has  the  middle  partition  removed. 
Put  the  plate  in  from  the  back  into  the  silver  plated 
kit,  film  side  down,  cover  with  two  small  and  one 
large  piece  of  blotting  paper,  and  then  slip  the  brass 
plate  on  top  of  these  and  into  the  spring  catch 
which  ordinarily  holds  the  plate.  Replace  the  back 
slide  and  expose  from  the  other  side.  Take  care 
not  to  handle  the  plate  holder  in  such  a way  as  to 
press  the  hard  rubber  slide  against  the  sensitive 
film.  Expose  in  the  view  camera  to  a negative 
illuminated  as  for  reduction  lantern  slides  (Exp.  8). 
Consult  the  bulletin  board  for  the  first  trial  expo- 
sure. 

Development. — Hold  the  plate  as  for  film  making 
and  pour  onto  the  exposed  film  enough  of  the  iron 


268 


PHOTOGRAPHY 


developer  to  cover  the  film  but  not  enough  that  any 
runs  off  (the  adhering  silver  nitrate  and  developer 
intensify  the  film  during  development).  Make  the 
developer  flow  gently  around  the  film  and  the  image 
should  appear  in  about  a minute.  When  develop- 
ment has  apparently  stopped,  rinse  the  plate  under 
the  tap,  and  fix  by  pouring  on  the  film  some  of  the 
saturated  hypo  solution.  Rinse  for  a minute  under 
the  tap  which  will  wash  the  exceedingly  thin  film. 
Thin  positives  are  quite  as  apt  to  be  from  insuffi- 
cient development  as  from  underexposure.  As 
with  gelatine  plates  overexposure  gives  a flat  pic- 
ture with  deposit  all  over  the  film.  They  are  easily 
intensified  either  before  or  after  fixing. 

Plain  Collodion  lodizer 

Pyroxline  9 g Ammonium  iodide  3 g 

Alcohol  (pure)  150  cc  Cadmium  iodide  3g 

Ether  (pure)  300  cc  Ammonium  bromide  1.3  g 

Alcohol  1 50  cc 

Mix  three  volumes  of  the  plain  collodion  with  one 
volume  of  the  iodizer  a few  days  before  use. 

Iron  Developer  Silver  Bath  | 

Ferrous  sulphate  30  g Silver  nitrate  90  g 

Acetic  acid  20  cc  Water  600  cc 

Water  500  cc  Nitric  acid  to  acid  reaction. 

Mount  two  of  the  best  of  your  collodion  plates  as 
lantern  slides,  and  submit  them  with  an  account  of 
the  day’s  work  including  description  of  any  special 
difficulties  and  successes. 


LABORATORY  MANUAL 


269 


ExPE)RIM^NT  21 

Pictures  Showing  Clear  Distance  and  Clouds. — 

Make  as  many  negatives  as  the  time  will  allow, 
choosing  views  showing  clouds,  blue  sky,  and  dis- 
tances. Use  the  color  screen  and  the  isochromatic 
or  polychromatic  plates.  Consult  the  bulletin  board 
for  screen  factor  and  the  table  in  the  text  (p.  220) 
for  the  speed  of  the  plates.  Also  consult  the  bul- 
letin board  for  suggestions  for  good  views.  It  will 
save  time  to  develop  in  the  tank. 

Report  on  each  plate  used  and  submit  negatives 
and  prints. 

Expe:rim]^nT  22 

Autochromes. — Each  box  holds  two  packages 
containing  two  plates  each.  In  the  package  these 
two  plates  are  placed  film  sides  toward  each  other 
with  two  pieces  of  thin  cardboard  in  between.  One 
piece  of  cardboard  should  be  kept  against  the  film 
(which  will  help  to  remember  which  is  the  film  side) 
and  the  plate  is  to  be  placed  in  the  kit  with  the  glass 
side  toward  the  lens  and  the  black  cardboard  still 
against  the  film.  This  is  all  to  be  done  in  the  dark. 
In  focusing  allowance  must  be  made  for  the  reversal 
of  the  plate  by  reversing  the  ground  glass  or  allow- 
ing for  the  thickness  of  the  Autochrome  glass  plate. 


270 


PHOTOGRAPHY 


The  exposure  is  to  be  made  through  a special 
color  screen.  As  a guide,  four  seconds  through  the 
screen  at  16  in  the  bright  summer  sunlight  will 
give  a good  exposure,  and  it  is  to  be  noted  that  the 
latitude  of  exposure  is  narrow. 

Develop  in  the  special  developer  (formula  below) 
for  2.5  minutes  between  16"^  and  20°  C.  The  whole 
development  is  to  be  carried  out  in  the  dark  with 
the  time  followed  by  the  light  of  the  red  lamp  facing 
away  from  the  tray  and  the  tray  covered. 

Rinse  off  the  developer  by  a gentle  stream  of 
water  from  the  tap  or  better  still  in  clean  water  in 
a tray,  as  the  tap  water  is  apt  to  spot  the  film  with 
solid  particles.  Then  immerse  in  the  potassium  per- 
manganate solution  and  go  out  into  the  light.  In 
3-4  minutes  all  the  negative  image  will  have  been 
dissolved  out  as  shown  by  the  transparency,  but  the 
plate  will  still  appear  a negative  by  reflected  light 
on  account  of  the  black  film  under  the  sensitive  film. 

Rinse  thoroughly  and  immerse  again  in  the  same 
developer  in  which  the  plate  was  first  developed  till 
it  all  goes  black,  3-4  minutes.  Rinse  again  for  3-4 
minutes  and  dry. 

After  the  film  is  thoroughly  dry  varnish  it  by 
pouring  a film  of  the  varnish  solution  on  the  plate 
like  a collodion  film  is  made.  Better  practice  it  on 
a glass  plate  first;  mount  the  plate  after  the  varnish 


LABORATORY  MANUAL 


271 

is  thoroughly  dry  as  for  a lantern  slide  so  that  the 
film  may  not  get  injured. 

The  film  is  very  thin  and  very  frail,  so  that  it  must 
not  be  touched  through  the  whole  process  by  any- 
thing but  the  solutions.  Breaks  through  to  the  color 
film  usually  show  a green  patch  around  the  break. 
If  specks  get  on  the  film  and  they  do  not  wash  off 
under  a gentle  stream  of  water,  leave  them  alone,  as 
attempts  to  remove  them  will  almost  inevitably 
break  the  film. 

In  case  the  finished  picture  after  final  develop- 
ment lacks  vigor,  it  may  be  intensified  with  advan- 
tage in  the  special  intensifier. 

The  composition  of  the  solutions  used  is  given  on 
the  printed  slip  in  each  box.  The  Lumiere  Com- 
pany also  issue  a booklet  giving  detailed  directions 
for  the  use  of  these  plates. 

Report  and  submit  each  plate. 

Experime:nt  23 

CARBON  PRINTING 

Sensitizing. — Brush  the  colored  gelatine  film  over 
with  the  Autotype  Spirit  Sensitizer  solution,  using 
some  of  the  solution  poured  out  in  a dish  and  the 
piece  of  flannel  tied  over  the  end  of  the  strip  of 
glass.  Brush  the  film  both  ways  lightly,  avoiding 
finger  marks,  and  when  the  solution  has  soaked  in 


272 


PHOTOGRAPHY 


(1-3  min.)  the  brushing  may  be  repeated.  Let  dry 
in  the  room  out  of  strong  light  for  half  an  hour 
and  then  keep  in  the  dry  box.  The  film  is  ready  to 
print  as  soon  as  dry  enough  not  to  stick  to  the 
negative. 

Printing. — The  dry  sensitized  film  will  usually  be 
rolled  up  and  quite  hard  and  must  be  unrolled  with 
care  and  slowly  to  avoid  cracking.  The  printing  is 
to  be  done  in  the  direct  sunlight  or  in  the  light  from 
the  electric  arc.  Since  the  image  is  not  visible,  the 
progress  of  the  printing  has  to  be  followed  by  the 
use  of  a strip  of  solio  paper  in  the  actinometer — a 
long  narrow  printing  frame  with  a series  of  num- 
bers increasing  in  density.  The  time  of  printing  is 
about  that  required  to  make  a solio  print  showing 
its  best  detail,  that  is,  perhaps  half  printed  for  a 
solio  picture.  As  an  approximation  try  5 minutes 
at  18  inches  from  the  arc  light.  The  printing  goes 
on  after  the  exposure  has  been  stopped,  so  if  the 
exposure  is  full  time,  the  development  should  take 
place  within  the  next  hour. 

Transfer.  — Soak  the  print  and  a sheet  of  single 
transfer  paper  in  water  at  room  temperature  till 
the  print  is  pliable.  Then  bring  the  two  faces  to- 
gether, under  water,  avoiding  air  bells ; lift  out,  and 
lay  on  a pad  of  blotters.  Squeeze  them  together  to 
get  rid  of  the  excess  of  water.  Put  another  pad 


LABORATORY  MANUAL 


273 

of  blotters  on  top  and  a weight  on  that  and  let  stand 
5>io  minutes. 

Develop  by  immersing  in  water  at  38-40°  C., 
using  the  enameled  iron  tray  on  the  iron  tripod, 
and  keep  the  water  at  this  temperature  by  regu- 
lating the  height  of  the  Bunsen  flame  underneath. 
After  a few  minutes’  soaking,  the  soft  gelatine 
will  be  found  oozing  out  between  the  edges  of  the 
print  and  transfer  paper.  They  are  now  to  be 
separated  gently  leaving  the  picture  on  the  trans- 
fer paper.  Soaking  longer  in  the  hot  water  will 
remove  the  rest  of  the  soluble  gelatine  or  the  proc- 
ess may  be  hastened  by  stirring  or  by  laving  the 
print;  the  most  detail  is  preserved  by  disturbing 
only  a little. 

Clearing.  — After  development  place  the  print  in  a 
5%  alum  solution  for  10-15  minutes  to  remove  any 
remaining  chromate.  Rinse  in  cold  water  and  allow 
to  dry. 

The  print  will  be  right-left  handed.  To  make 
them  correct  requires  another  transfer  which  modi- 
fies the  above  procedure  somewhat. 

Expe^rim^nT  24 

Characteristic  Curve. — Cut  down  one  of  the  or- 
dinary plates  so  as  to  fit  the  3^  x 4^^  holder  for 
the  Chapman  Jones  Plate  Tester.  Scratch  the  glass 


274 


PHOTOGRAPHY 


with  a wheel  cutter  and  the  film  will  tear  easily. 
Expose  the  plate  behind  the  exposing  plate  for  30 
seconds  to  the  candle  after  the  candle  has  burned 
for  several  minutes.  Finish  as  usual.  When  dry 
measure  the  densities  of  the  spots  from  1-20  in  the 
photometer.  Read  over  the  description  of  the 
photometer,  page  280,  and  get  from  the  instructor 
a negative  similar  to  yours  to  use  in  the  right  hand 
window.  The  densities  of  the  numbered  spots  on 
the  exposing  plate  are  supposed  to  increase  by  equal 

additions,  each  addition  absorbing  the  light  get- 

V 2 

ting  to  it  so  that  the  exposures  decrease  by  a factor 
of  V2.  Hence  plot  the  numbered  spots  at  equal  dis- 
placements on  a horizontal  axis  and  the  densities  of 
each  spot  vertically  and  draw  the  curve  through  the 
resulting  points. 

Submit  negative,  numerical  data,  and  curve  with 
your  report. 

Finishing  Up  Work 

In  connection  with  the  grade  for  the  semester, 
will  you  please  leave  in  your  locker  for  inspection 
the  following  minimum  requirement : 

Complete  reports  to  date 

Album  with  the  full  list  of  prints,  including  also  four 
P.  0.  P.  prints  and  two  carbon  prints. 

4 Lantern  slides 
2 Collodion  lantern  slides 
Autochromes 
Two  best  ’enlargements 


LABORATORY  MANUAL  275 

The  negatives  are  not  needed.  After  inspection 
all  these  are  your  personal  property.  The  labora- 
tory keys  must  be  surrendered  when  the  above  re- 
sults of  the  semester's  work  are  taken  into  private 
possession.  The  laboratory  will  be  open  for  this 
at  the  times  posted  on  the  bulletin  board. 

The  written  examination  is  supposed  to  cover  all 
the  work  of  the  laboratory  and  of  the  lectures. 


APPENDIX  I 


Laboratory  Apparatus 

Required  for  each  dark  room  which  accommo- 
dates two  students  at  one  time. 

I camera,  4 x 5,  in  case 
4 plate  holders,  4x5,  with  hard  rubber  slides 
I focusing  cloth 
6 printing  frames,  4x5 
I Centigrade  thermometer 
I camePs-hair  brush,  flat,  i inch 
I circular  print  washer 
I plate  washer,  for  4 x 5 and  3M  x 4 plates 
I measuring  glass,  4 oz. 

1 tripod 

4 deep  hard  rubber  trays,  43^  x 5J4 

2 deep  hard  rubber  trays,  10  x 12 

1 print  stick 

2 tray  covers 
I glass  funnel 

I ruby  safe  light  for  plates 
I hanging  tipless  ruby  incandescent  lamp 
I large  sink,  3 faucets 
Group  of  lockers,  10  or  15  per  room 
Desk,  top  preferably  covered  with  sheet  lead,  to  be  water 
tight. 

I drying  cabinet.  The  simplest  form  is  a box  with  a 
light  tight  door  and  large  enough  to  hold  a plate  dr)dng 
rack  with  a dozen  plates.  It  has  to  be  provided  with  tor- 
tuous passages  for  inlet  and  outlet  of  air  and  some  arrange- 
ment for  forcing  the  air  through. 

In  addition  to  the  above  each  locker  needs  a collapsible 
drying  rack  to  hold  one  dozen  negatives. 

276 


APPENDIX 


277 


Special  Apparatus 

Required  Set  for  Each  Two  Dark  Rooms 

I view  camera,  x 8j/^  standard  type  with  long  exten- 
sion bellows,  large  removable  lens  board,  and  vertical  ad- 
justment of  lens.  The  plate  holder  carrier  must  be  square 
and  removable  so  that  it  may  be  placed  either  edge  ver- 
tical, or  replaced  by  other  parts. 

I special  back  for  the  view  camera  having  ground  glass 
and  opening  for  4x5  plate  holder.  It  requires  also  the 
sliding  screen  described  in  Exp,  12. 

I high  grade  modem  lens,  to  cover  sharply  a 4 x 5 plate. 

I incandescent  tungsten  nitrogen-filled  lamp,  100  watt 
locomotive  headlight  type  with  concentrated  filament. 

I water  cell  arranged  to  carry  a 6-inch  condensing  lens 
on  each  face. 

I light  tight  box  for  holding  the  above  lamp  and  water 
cell.  It  requires  a door  for  adjustment  and  an  opening,  the 
inside  of  which  is  shaped  to  fit  the  backs  for  the  view 
cameras,  and  the  outside  of  which  is  fitted  to  receive  the 
opening  in  the  back  of  the  view  camera  itself. 

I back  for  view  camera  fitted  with  simple  adjustable  slit. 

I back  for  the  view  camera  made  with  4x5  opening  for 
carrying  a ground  glass  and  4x5  negative. 

I low  grade  achromatic  lens,  30  or  40  cm  focal  length 
and  3 or  4 cm  diameter. 

I replica  grating,  second  quality,  3 or  4 cm  square. 

I color  screen,  fairly  dark,  to  give  unmistakably  clear 
distances  and  strong  clouds. 

I drying  box.  An  ordinary  bread  box  fitted  with  a glass 
tray  of  calcium  chloride  covered  with  brass  wire  gauze. 

1 Watkins  Bee  meter. 

8 kits,  4 X 5 to  334  X 4. 

2 doz.  push  pins. 

I small  iron  pot  for  cleaning  the  glass  in  making  collodion 
slides. 

1 developing  tank,  4x5,  reversible. 

2 enamel  trays  for  holding  hot  water  for  carbon  print- 

ing. 

Trimming  board,  one  or  two  for  the  whole  laboratory. 

Chapman  Jones  Plate  Tester,  made  by  Sanger  Shepherd 


278  PHOTOGRAPHY 

and  Co.,  Holbom,  London,  W.  C.,  England.  One  or  two  for 
the  laboratory. 

Scales  and  graduates  for  making  up  solutions. 

I plate  holder  for  collodion  plates,  made  by  removing 
the  central  septum  from  an  ordinary  4x5  plate  holder. 
The  X 4 plate  is  held  in  a brass  kit  which  is  silver  plated 
and  is  arranged  to  be  filled  from  the  back  and  needs  no  fas- 
teners, being  held  by  some  blotter  and  a brass  plate,  4x5, 
with  a finger  hole.  See  Exp  19. 

I dipping  bath  for  the  collodion  plates.  Anatomy  mu- 
seum jars  of  suitable  size  are  fitted  into  a wooden  box  with 
a hinged  cover  and  with  feet  arranged  so  that  the  box  stands 
with  a slight  tilt.  The  plates  are  carried  on  a silver  wire 
frame  fastened  together  with  silver  solder. 

I small  single  filament  Nemst  glower. 

Supplies  per  Student 

These  quantities  are  necessarily  very  approximate  as  they 
vary  materially  from  year  to  year. 

1 album  for  4 X 5 prints 

2 pads,  report  blanks  (50  pages) 

JDry  plates,  3 doz.  ordinary 

I doz.  isochromatic 
3^  doz.  polychromatic 

12  doz.  developing  paper,  4x5,  medium  contrast,  semi- 
matt,  thin  paper. 

I manilla  paper  report  binder 

I manilla  paper  envelope  in  which  to  hand  in  reports,  etc. 
Paper  toweling. 

I doz.  enlarging  paper,  10  x 12 

1 doz.  printing  out  paper 

2 doz.  lantern  slide  plates 

I doz.  lantern  slide  cover  glasses 

I doz.  lantern  slide  matts. 

3^  doz.  carbon  tissue,  4x5,  assorted  colors. 

}/2  doz.  single  transfer  mount,  4x5.  These  last  two  items 
it  pays  well  to  buy  in  the  roll  and  cut  up.  Autotype  tissue 
and  supplies  may  be  bought  through  George  Murphy,  57 
East  Ninth  Street,  New  York. 

3^  pkg.  lantern  slide  binding  strips. 

34  blotter,  photographic,  19  x 24. 


APPENDIX 


279 


H ^oz.  blue  print  paper,  4x5. 

2 sheets  8 x 10,  good  linen  paper  for  blue  print  sensi- 
tizing. Size  with  starch  or  better  gelatine. 

2 Autochromes,  334  x 4. 


Chemicals  per  Student 


Acetic  acid,  2 oz. 

Alcohol,  absolute,  i oz. 
Alum,  powdered,  2 oz. 
Borax,  34  oz. 

Ether,  i oz. 

Ferrous  sulphate,  2 oz. 
Glycerine,  34  oz. 

Gold  chloride,  2 grains 
Hydrochinon,  i oz. 

Sodium  hyposulphite,  4 lbs. 
Mercuric  chloride,  i oz. 


Metol,  34  oz. 

Potass,  bromide,  i oz. 

Potass,  carbonate,  dry,  2 oz. 
Potass,  ferricyanide,  i oz. 
Potass,  oxalate,  4 oz. 
Pyrogallic  acid,  i oz. 

Silver  nitrate,  ^ oz. 

Sodium  carbonate,  dry,  i lb. 
Sodium  sulphite,  dry,  i lb. 
Spirit  sensitizer  for  carbon 
printing,  i oz. 


and  small  quantities  of  the  following: 


Ammonia  liq. 
Ammonium  bromide 
Ammonium  iodide 
Benzol 

Cadmium  iodide 
Ferric  ammoniiun  citrate 
Litmus  paper 


Negative  cotton 
Oxalic  acid 

Potass,  metabisulphite 
Potass,  permanganate 
Rubber  cement 
Sodium  sulphide 


APPENDIX  II 


Photome^tdrs 

There  are  several  types  of  photometers  on  the 
market  but  they  are  expensive,  often  elaborate  in- 
struments and  hardly  suited  to  this  work.  A mod- 
ification of  Hurter  and  Driffield’s  original  arrange- 
ment, making  use  of  the  inverse  square  law,  can  be 
made  sufficiently  accurate,  simple,  and  inexpensive. 
Such  an  arrangement  is  described  by  W.  B.  Fergu- 
son.^ His  comparison  arrangement,  however,  is  not 
satisfactory  and  a much  better  arrangement  is  sub- 
stituted here. 

Two  ways,  a ^ inch  brass  tube  and  a ^ by 


brass  strip,  are  screwed  parallel  to  each  other  and 
four  inches  apart  on  a piece  of  seasoned  oak 
7 X I X 6o  inches.  Fig.  51  is  a diagram  of  the  gen- 
^ Photographic  Journal,  Vol.  52,  page  283. 


APPENDIX 


281 


eral  arrangement.  On  these  ways  are  carried  two 
lamps  and  between  them  the  box  with  the  photom- 
eter comparison  head.  They  are  held  in  line  by  a 
slotted  piece  of  brass  tube  sliding  on  the  inch> 


Fig.  52.  Central  Box  of  Bench  Photometer. 

guide.  The  most  satisfactory  lamps  available  are 
the  nitrogen-filled  concentrated  tungsten  filament 
locomotive-headlight  incandescent  lamps,  and  the 
100  watt  size  will  give  light  enough  to  read  all  the 
ordinary  densities.  They  usually  run  on  lower  volt- 
age than  the  lighting  circuit  so  that  they  are  prob- 
ably best  connected  in  series  with  each  other  and 


282 


PHOTOGRAPHY 


with  sufficient  resistance  to  give  them  their  rated 
current.  Automobile  headlight  lamps  of  lower  wat-. 
tage  or  s ter eopt icon  lamps  of  higher  are  available. 
The  left  hand  lamp  has  a pointer  on  its  carriage  in 
the  same  vertical  plane  as  the  filament  and  the  posi- 
tion of  the  lamp  is  read  on  a scale  placed  parallel 
to  the  brass  tube.  An  ordinary  meter  stick  will 
serve  but  the  readings  require  a lot  of  calculating 
which  may  be  avoided  by  the  use  of  a logarithmic 
scale.  Below  is  given  the  algebra  from  which  a 
scale  may  be  constructed  which  will  read  densities 
directly. 

In  the  center  of  the  board  is  set  up  a rectangular 
box  6x8x3  inches,  Fig.  52.  In  the  6x8  face 
toward  the  lamps  are  round  holes  0.75  inches  in 
diameter  set  at  the  height  of  the  filaments.  Held 
in  front  of  each  hole  by  strap  springs  from  each 
corner  of  the  box  is  a disc  of  wood  with  a 0.75  inch 
hole  in  its  center,  which  is  covered  on  the  side  next 
the  box  by  a piece  of  opal  glass  (O).  The  nega- 
tive slips  under  this  disc  till  the  place  required  is  in 
line  with  the  holes  and  the  springs  must  be  suffi- 
ciently strong  to  hold  the  negative  from  falling. 
Supported  from  the  inside  rear  wall  of  the  box  are 
a pair  of  right-angled  glass  prisms  set  as  in  the 
diagram,  which  serve  as  mirrors  so  that  one  look- 
ing from  above  sees  two  images  in  contact,  one  from 
each  side  of  the  box.  No  viewing  lenses  are  re- 


APPENDIX 


283 

quired,  merely  a tube  to  place  the  eye  about  the 
distance  of  distinct  vision  and  to  protect  from  stray 
light.  The  line  separating  the  two  images  may  be 
made  almost  a geometric  line  and  the  conditions 
for  comparison  of  the  brightness  of  the  two  images 
are  excellent.  Of  course  the  whole  inside  of  the 
box  must  be  dead  black  and  otherwise  completely 
closed. 

An  arrangement  for  varying  the  visible  area  of 
the  negative  is  necessary  for  adjusting  the  nega- 
tive in  position  and  for  arranging  always  to  observe 
only  a uniform  area.  A piece  of  brass  with  a verti- 
cal V-shaped  opening  is  arranged  to  slide  in  grooves 
inside  the  box  in  front  of  each  of  the  openings. 
The  result  is  that  the  observer  sees  two  triangular 
images  with  bases  together,  and  the  size  of  the 
triangles  may  be  readily  adjusted  by  moving  the 
brass  V by  a rod  projecting  through  the  top  of  the 
box.  To  read  the  densities  of  the  spectrum  plates 
another  brass  slider  is  arranged  to  cut  off  the  apex 
of  the  triangular  image  so  that  narrow  strips  across 
the  spectral  band  may  be  compared. 

Set  the  apparatus  as  in  the  diagram  Fig.  51, 
except  that  A is  at  the  zero  and  C at  the  infinity  of 
the  scale.  Insert  at  E a negative  whose  density  is 
such  that  the  lamp,  D,  stays  on  the  ways  when  the 
first  adjustment  is  made  by  moving  D till  the  two 
spots  are  the  same  brightness.  The  better  the  color 


284 


PHOTOGRAPHY 


match  of  the  negatives  at  C and  E the  more  reliable 
are  the  settings.  To  read  total  density  adjust  D so 
that  the  illumination  matches ; then  insert  the  plate 
at  C and  move  A till  the  illumination  matches  again, 
and  the  scale  reading  will  give  the  total  density. 
To  read  the  density  less  fog,  glass,  and  gelatine, 
place  the  unexposed  spot  in  C while  making  the  first 
setting  of  D and  make  the  second  setting  as  above 
and  the  scale  reading  will  be  the  density  less  fog, 
glass,  and  gelatine. 

If  the  density  of  the  spot  which  it  is  desired  to^ 
read  is  so  great  that  A can  not  be  moved  near 
enough  to  C to  make  a match,  it  becomes  necessary 
to  substitute  a spot  of  intermediate  density.  Read 
its  density  as  above,  and  with  all  parts  in  the  final 
positions  of  the  reading,  move  A back  to  zero. 
Readjust  for  equality  by  moving  D and  if  neces- 
sary changing  the  negative  E,  and  proceed  with  the 
reading  of  the  original  spot.  In  this  way  as  many 
stops  may  be  put  between  as  are  necessary  to  bring 
the  final  reading  on  the  scale,  and  the  density  of  the 
spot  in  question  will  be  the  sum  of  this  series  of  dif- 
ferences. With  each  shift  of  the  lamp,  A,  back  to 
zero,  the  brightness  of  the  image  falls  off,  thus  set- 
ting a limit  to  the  series  unless  brighter  sources  can 
be  substituted. 

Let  i be  the  brightness  of  the  image  when  matched 


APPENDIX  28s 

at  X as  above.  Then  in  the  first  setting  where  there 
is  no  absorption 

• = ^ 

^ OC2 

where  S is  the  strength  of  a source  such  that  when 
placed  at  O it  would  give  the  observed  brightness  at 
C if  there  was  no  loss  of  light  between  O and  C.  In 
the  second  setting  the  image  brightness  remains  the 
same,  being  matched  with  the  same  brightness  at  E, 
but  the  incident  brightness  (I)  is  now 


and 

^ “ xc^ 

„ I S , OC2  oa 

Opacity  = i - s - =xa 

Whence 

D>  = lo&[  = 2 (log.OC  - log.XC) 

Or 

D = ^1-  = 2 (logioOC  - logioXC) 

2.303 

For  this  instrument  OC  is  70  cm  and  if  D be  set 
equal  to  any  particular  value,  a corresponding  value 
for  XC  may  be  calculated  from  this  last  equation. 
A suitable  series  of  values  of  XC  are  then  meas- 
ured off  on  the  scale  and  each  marked  with  its  cor- 
responding value  of  D.  It  is  to  be  noted  that  is 
the  density  in  absolute  units  and  D the  density  in  an 
arbitrary  unit  involving  the  factor  for  the  base  10. 
This  arbitrary  unit  is  the  one  usually  employed. 


286 


PHOTOGRAPHY 


To  obtain  a value  for  y (gamma),  that  is,  the 
slope  of  the  characteristic  curve,  expose  a plate  so 
that  the  exposures  change  by  a definite  known  fac- 
tor, most  simply  by  the  use  of  a plate  with  a graded 
series  of  densities  (see  Exp.  23).  Develop  the  whole 
plate  alike.  Measure  the  D (always  less  fog,  etc.) 
and  plot  D as  function  of  the  logarithms  of  the 
exposure,  that  is,  plotting  equal  factors  of  expo- 
sure as  equal  lengths.  Choose  two  points  near  the 
ends  of  the  straight  part  of  this  characteristic  curve. 
Calling  these  two  points  i and  2,  it  will  be  evident 
from  Art.  26,  and  the  above  discussion  that 


and  the  ratio  E1/E2  is  independent  of  the  unit  in 
which  the  E’s  are  expressed,  so  that  any  convenient 
unit  may  be  used.  It  is  not  necessary  to  determine 
the  inertia,  i,  nor  is  it  necessary  that  the  extension 
of  the  straight  part  of  all  the  characteristic  curves 
should  cut  in  a point  either  on  or  off  the  axis. 

To  obtain  the  development  time  for  any  value  of  y, 
plot  several  values  of  y obtained  as  above,  as  func- 
tions of  the  time  of  development  used  to  obtain  each 
value;  draw  a smooth  curve  through  these  points 
and  from  this  curve  can  be  read  the  time  of  devel- 
opment for  any  desired  value  of  y,  the  most  useful 
value  being  unity. 


Di-Dl 


^ log,E2-log,Ei 


APPENDIX 


287 

To  obtain  the  speed  of  a plate,  make  a series  of 
known  exposures  and  develop  it  for  the  above  de- 
termined time  for  7 equal  one.  Plot  the  character- 
istic curve  and  read  the  value  for  the  inertia,  which 
will  be  the  reciprocal  of  the  speed.  For  absolute 
values  of  the  speed,  the  exposures  must  be  ex- 
pressed in  candle-meter-seconds  and  the  light  source 
must  be  described.  Where,  however,  the  speeds  are 
to  be  used  for  comparison  with  other  speeds  ob- 
tained in  the  same  way,  any  unit  for  the  exposure 
may  be  used.  With  plates  which  have  different 
color  sensitiveness,  the  character  of  the  light  source 
may  have  a very  material  effect  on  the  relative 
speeds. 


288 


PHOTOGRAPHY 


Order  Slip 

Name Lab.  No 

Date 

App.  (to  be  retiimed) 


Supplies 


Photography  Laboratory  Record 


Name 

Negative 

Number 

Subject 

Character  of  subject 

Date  and  hour  of  day 

Character  of  light 

Time  of  exposure 

Stop  used 

Brand  of  plate 

Color  screen 

Exp.  No 

Developer 

Time  of  development 

Character  of  negative 

Temp 

Print 

Time  of  printing 

Brand  of  paper 

Character  of  light 

Date 

Distance  from  light 

Purpose  of  the  work 

Conclusions. 


INDEX 


Aberrations,  definition  of,  150 
Abney,  13 

Absorption  curves,  77 
Absorption  of  light,  law  of,  21 
Achromat,  long  focus,  253 
Achromatic  lens,  definition  of, 

155 

secondary  correction,  155 
Achromatism,  154 
Acid,  hypo  form,ula,  236 
Addition  method,  179 
Aerial  perspective,  203 
Age  of  plate,  47 
Age  of  plates,  fog,  106 
Air  bells,  112 

Albumenochloride  paper,  125 
Alum  in  hypo,  iii 
Ammonium  persulphate,  107 
Anethol,  194 
Angle  of  view,  163,  210 
Apparatus,  in  dark  room,  232 
in  locker,  231 
list,  276 
special  list,  277 

Appendix  on  development,  219 
Artificial  latent  image,  90 
Astigmatism,  159 
Autochromes,  187,  269 
development  of,  188 
exposure  of,  188 
latitude  of,  189 

289 


Autochromes,  manufacturing, 
188 

reversal  of,  188 
Autotype,  137,  180, 

Autotype  spirit  sensitizer,  271 
Autotype  supplies,  278 
Axis  of  lens,  146,  213 

Backed  plates,  116 

Backing  for  plates,  246 

Balance,  208 

Bancroft,  90 

Barrel  distortion,  161 

Bench  photometer,  23,  281 

Bitumen,  4 

Black  spots,  1 12 

Bleacher  for  sepia  toning,  265 

Bleach-out  process,  193 

Bloch,  Olaf,  120 

Blue  haze,  204 

Blue  print,  124,  258 

Blue  print  paper,  formula  for, 

259 

Blurring,  212 

Boiling  method  for  emulsions, 

13 

Bolton,  9 

Bromide  paper,  127,  263 
Bubbles  in  film,  118 
Bubbles  in  lenses,  163 
Burroughs  Wellcome,  224 


290 


INDEX 


Camera  obscura,  3 
Candle -meter -second,  defini- 
tion of,  33 

Carbon,  three  color,  180 
Carbon  printing,  135,  271 
Carbon  tissue,  137 
Carey  Lee,  93 
Celluloid,  7 

Celluloid,  colored  sheets, 
190 

Center  of  picture,  geometric, 
214 

Centrifuge  method,  16 
Chapman  Jones,  102 
method  of  intensification 
of,  102 

plate  tester,  273,  277 
Characteristic  curve,  273 
definition  of,  33 
effect  of  development  on 
slope  of,  43 

effect  of  potassium  bromide 
on,  45 

method  of  making,  36 
slope,  43 

Character  of  subject,  68 
Chemical  fog,  25 
Chemicals  per  student,  279 
Chiaroscuro,  207 
Chromatic  aberration,  153 
Chondrin,  ii 

Cinemetograph  films,  223 
Classification  of  color  photo, 

173 

Cleanliness,  112 
Cleaning  fingers  of  silver 
stain,  268 

Cleaning  glass,  15,  265 


Cloud  pictures,  269 
Clouds,  88,  202 
Collodion,  7 
plain  formula  for,  268 
wet,  7 

Collodion  emulsions,  9 
Collodion  process,  7,  265  ^ 
washed,  9 

Colloidal  solutions,  95 
Colloids,  II 
Color,  73 
impure,  73 

Color  contrast,  85,  87 
Color  pure,  73 
Color  photo,  173 
Color  records,  179 
Color  screen,  252 
for  autochromes,  188 
for  clouds,  202 
for  colored  subjects,  205 
for  distance,  204 
Color  screen  plate,  223 
Color  screens,  84 
Color  sensitiveness,  72,  252 
due  to  dyes,  75 
of  AgBr,  Agl,  AgCl,  75 
Color  sensitiveness  curves,  73, 
256,  257 
for  dyes,  81 
for  mixture  of  dyes,  82 
Color-density  curve,  256,  257 
Colored  subject,  204,  252 
Colors  for  3 color  prints,  181 
Coma,  152 

Composition,  picture,  197 
Constant  density  ratios,  29 
Construction,  of  lens  images, 

147 


INDEX 


291 


Contact  lantern  slides,  242 
Contrast,  30,  41,  47,  50 
color,  85,  87 
Contrast  papers,  248 
Control  of  development,  47 
Convertible  lens,  169 
Correct  exposure,  239 
period  of,  37 
Crown  glass,  155 
Curvature  of  field,  159 
Curves,  color  sensitiveness,  73 

Daguerre,  5 

Dark  room  apparatus,  232 
Davy,  4 

Daylight  table,  70 
Dense  negatives,  103 
Density,  by  photometer,  284 
definition  of,  22 
of  gelatine,  25 
of  glassplate,  25 
growth  with  time,  28 
Density  measurements,  23 
Density  ratios,  29 
Density-color  curve,  79 
Density-development,  relation 
to,  29 

Density-opacity  relation,  22 
Depth  of  focus,  165,  21 1 
Detail,  215 

Developer,  ferrous  oxalate,  27 
Developer  for  paper,  formula, 

237 

Developers,  55 

Developing  collodion  plates, 
267 

Developing  factor,  44,  45,  51 
condition  affecting,  45,  46 


Developing  paper,  127,  237 
effect  on  characteristic 
curve,  43 

effect  of  time  of,  26 
Development-density  relation, 
29 

manipulation  during,  31 
methods  of  control  of,  47 
pictorial  considerations,  214 
by  tank,  32 

Development  speed  list,  220 
Development-temperature  co- 
efficient, 48 

Development  time  by  photom- 
eter, 286 
Diaphragm,  151 
Diffraction,  144 
Dipping  bath,  278 
formula,  268 

Directions,  general,  for  labo- 
ratory, 231 
Dispersion,  155 
secondary,  156 

Distance  of  distinct  vision,  209 
Distance  pictures,  269 
Distinct  vision,  distance  of,  209 
Distortion,  160 
barrel,  161 

from  inclined  plate,  213 
from  short  focus,  210 
pin-cushion,  161 
Dodging,  104,  106 
Double-coated  plates,  116,  246 
Doublet  lens,  160 
Draper’s  Law,  77 
Driffield,  19 
Drying  cabinet,  256 
Drying  film  by  alcohol,  119 


• 292 


INDEX 


Drying  negatives,  119 
Dry  plate,  10 
age  of,  47 

Dry  plate  making,  13 
boiling  method  for,  13 
Dry  plate  speed,  64 
Dufay,  191 
Dust  in  the  air,  203 
Dust  marks,  iii 
Dyeing  plates,  256 
Dyes,  75 

Eastman  Kodak,  224 
Eder,  92 

Eder  Handbuch,  225 
Effect  of  time  of  development, 
26 

Efficiency  of  shutters,  60 
Ellis,  102 

Emulsion,  ammonia  method, 
16 

coating,  16 

centrifuge  method,  16 
washing,  15 
Emulsions,  collodion,  9 
Enlarging,  262 
Enlargement,  contrast,  262 
exposure,  263 
Examination,  275 
Exposing  collodion  plates,  267 
Exposing  paper,  238 
Exposure,  definition  of,  57 
pictorial  considerations,  214 
Exposure  calculators,  70 
Exposure-density  relation,  33 
Exposure  meters,  70 

Eactor,  development,  44,  45 
Eactorial  development,  241 


Eactor ial  system,  Watkins,  50 
Eading  of  latent  image,  90 
Earmer  reducer,  251 
East  lens,  168 
Ferguson,  280 

Ferric  ammonium  citrate,  124 
Ferricyanide  hypo  reducer, 

251 

Ferric  oxalate,  133 
Ferrous  oxalate,  123 
Ferrous  oxalate  developer,  27, 
244,  250 

Ferrous  sulphate  solution,  245 

Film,  microscope  study,  107 

Film  cameras,  17 

Film  side,  to  tell,  234 

Film  thickness,  52 

Films,  17,  223 

Finger  marks,  113 

Fish  glue,  140 

Flare,  162 

Flint  glass,  155 

F numbers,  63 

Focal  length,  definition  of,  147 
of  lens,  208 
Focus,  146 
virtual,  146 
Focusing,  21 1 
Fog,  25,  51 
chemical,  25 
definition,  etc.,  of,  104 
Fog  due  to  age,  106 
Fog  strip,  26 
Fogging  exposure,  67 
Foreground,  201 
Forming  collodion  film,  £66 
Foxlee,  265 
Frilling,  no 


INDEX 


293 


Full  scale  of  paper,  214 
Fuzzy  effects,  215 

Gallic  acid,  6 
Gamma,  44 
by  photometer,  286 
Gamma  infinity,  ^00,  44 
Gas  light  paper,  127,  237 
Gelatine,  ii 
absorption  of  dyes,  184 
hard,  ii 

Gelatine  chloride  paper,  125, 
260 

Gelatine  dryplate,  10 
Gelatine,  solubility  in  alum,  12 
Gelatine  substratum,  16 
General  directions  for  labora- 
tory, 231 

George  Murphy,  278 
Ghost,  162 
Glass,  cleaning,  15 
Glutin,  II 

Glycerine  and  paper  backing, 
246 

Gold  chloride  solution,  for- 
mula, 261 
Good  pictures,  195 
Grain  of  plate,  68 
Grating,  73 
replica,  254 
Greasy  ink,  192 
Grease  spot  photometer,  23 
Growth  of  density,  28 

Halation,  104,  114,  246 
Halftone,  139 
H & D,  225 
number,  66 


Hartmann,  198 
Haze,  blue,  204 
Herschel,  6 

Heyde’s  action  photometer,  71 
High-lights,  42 

Howard  Farmer  reducer,  107, 

251 

Hurter  and  Driffield,  19,  23, 
65,  128,  280 
Huse  and  Nietz,  107 
Hydrochinon  developer,  for- 
mula for,  245 
Hypo  for  p.  o.  p.,  261 
Hypo  acid,  formula  for,  236 

Ilford,  224 

Illumination  for  microphotos, 

258 

Image,  size  of,  149 
Image  formation,  146 
Impressionist  effects,  212,  21 
Impure  color,  73 
Incident  light,  21 
Inclined  plate,  distortion  of, 
213 

India  ink,  25 
Inertia  “i,’'  225 
Inertia  of  plate,  65 
Intellectual  problem,  217 
Intensification,  33,  loi,  247 
Intensifier,  Chapman  Jones, 
102 

mercury,  102 
Interference  of  light,  176 
lodizer,  formula  for,  268 
Iron  developer,  formula  for, 
268 

Irradiation,  117 


m 


294  INDEX 


Isochromatic  plate,  252,  255 
Joly,  186,  190 

Jones,  Nutting  and  Mees,  128 

Krayn,  190 
Kromskop,  180 

Laboratory,  general  directions 
for,  231 

Laboratory  apparatus,  276 
Lambert,  199 

Lamp  for  photometer,  281 
Lantern  slide,  developer,  244, 

245 

plates,  243 
printing,  243 
Lantern  slides,  121 
contact,  242 
reduction,  245 
Latent  image,  43,  89 
Latent  image  fading,  90 
Latent  image  theories,  91 
Latitude,  52,  53,  58 
of  autochromes,  189 
of  paper,  130 

Law  of  absorption  of  light,  21 
Lens,  145 
angle  of  view,  21 1 
doublet,  160 
focal  length  of,  208 
wide  angle,  165 
Lens  axis,  213 
Lenses,  apochromatic,  158 
types  of,  145 

Light  absorption,  law  -of,  21 
Lighting,  69,  206 
Light  struck,  105 


Line  screen,  139 
Lippmann  process,  173,  174 
Local  reduction,  109 
Locker  apparatus,  231 
Long  focus  achromat,  253 
Lumiere  and  Seyewetz,  105 
Luminosity  curve,  80 

MacDonough,  190 
Manipulation  during  develop- 
ment, 31 
Mask,  216 
Mass  effect,  207 
Matt  surfaces,  128 
Maximum  contrast,  51 
Maximum  contrast  in  prints, 
248 

Maxwell,  178 
Mees,  C.  E.  K.,  54,  7,  8 
Mees  and  Piper,  56 
Melting,  162 
Meniscus  lens,  152,  169 
Mercury  chloride  intensifica- 
tion, 102 

Mercury  intensifier,  249 
Meters,  exposure,  70 
Metol  hydro,  see  paper  devel- 
oper, 237 

Microphotography,  257 
Microscopic  study  of  film, 
107 

Mordant,  185,  191 
Mosaic  screen  making,  190 
Mosaic  screen  plates,  sum- 
mary, 192 

Mosaic  screen  processes,  186 

Mounting,  216 

Moving  picture  films,  223 


INDEX  295 


Nernst  lamp,  253,  254 
Niepce,  4 
Nodal  points,  147 
Notan,  207 

Numbering  of  stops,  63 

Opacity,  definition  of,  22 
Opacity-density  relation,  22 
Order  slip,  288 
Overdevelopment,  104 
Overexposed  negative,  251 
overexposure,  103 
period  of,  42 

Oyster  shell  markings,  1 18 

Paget  Prize  Plate,  192 
Panchromatic  plate,  83 
Paper  developer,  formula  for, 

237 

Period,  of  correct  exposure, 
37 

of  overexposure,  42 
of  reversal,  43 
of  underexposure,  40 
Perfect  negative,  20,  39 
conditions  for,  40 
Perspective,  165,  210 
aerial,  203 

Petzval  portrait  lens,  169 
Photo-engraving,  139 
Photohaloids,  93 
Photometer,  23,  280 
Photometer  algebra,  285 
Photometer,  bench,  23 
Physical  modification  theory, 

91 

Pictorial  pictures,  196 


Picture,  definition  of,  195 
Picture  composition,  197 
Pictures,  pictorial,  196 
record,  196 

Pigment  color  processes,  173 
Pinachrome,  81,  85 
Pinacyanol,  256 
Pinatype,  181,  183 
Pin-cushion  distortion,  i6i 
Pinhole  image,  142 
Plain  collodion,  formula,  268 
Plate,  dry,  10 

developer,  formula  for,  236 
grain,  68 
inertia,  65 
speed,  64 

speed  by  photometer,  287 
speed  numbers,  table  of, 
225 

Plate  speeds,  list  of,  220 
Platinotype,  133 
Polychromatic  plate,  255,  257 
Poore,  198,  208 
P.  O.  P.  hypo,  formula  for, 
261 

Portrait,  200 

Potassium  bromide,  effect  on 
characteristic  curve,  45 
Potassium  cyanide,  8 
Potassium  iodide,  in  develop- 
er, 1 14 

Potassium  oxalate,  135 
Potassium  ferricyanide  re- 
ducer, 251 

Preface  to  manual,  229 
Preliminary  exposure,  67 
Pressure  marks,  92,  113 


INDEX 


296 

Primary  spectrum  colors,  179 
Printing,  238 

pictorial  considerations,  214 
Printing  colors,  181 
Printing  out  paper,  125,  260 
Printing  press,  186 
Prints,  maximum  contrast,  248 
Properties  of  developing  pa- 
pers, 128 
Pure  color,  73 
Pyro,  234 
stock  formula,  236 
Pyroxyline,  7 

Rapid  rectilinear  lens,  161,  168 
Reactions  with  dry  plates,  18 
Record  pictures,  196 
Rectilinear  lens,  161 
Reducer,  ammonium  persul- 
phate, 107,  108 
ferri-cyanide,  107 
Howard  Farmer,  107,  251 
potassium  permanganate, 
107 

Reducers,  classification  of,  107 
Reduction,  251 
local,  109 

Reduction  lantern  slides,  245 
Reflex  camera,  167 
Replica  grating,  73,  254 
Report,  general  directions,  233 
Report  blanks,  233,  288 
Restrainer,  36 
Reticulation,  249 
Reversal,  37,  43 
Robinson,  198 
Rodinal,  224 
Rotary,  180 


Sagging  of  film,  119 
Sanger-Shepherd,  180 
Scattering  by  dust,  204 
Scheffer,  68,  107,  115 
Scheiner,  225 
Schott,  158 
Scheele,  3 

Screening  action  of  dyes,  83 
Screens,  color,  84 
Secondary  dispersion,  155 
Sensitiveness,  14 
Sensitizing  collodion  plates, 
266 

Sepia  toning,  264 
Shadow  detail,  42 
Shadows,  42 

Sheppard  and  Mees,  20,  90, 105 
Shutter,  consistent,  59 
efficiency,  60 
markings,  60 
reliability,  60 
testing,  60 

Shutters,  types  of,  58 

Silver  bath,  formula,  268 

Silver  bromide,  4 

Silver  grain  hypothesis,  92 

Silver  grain  theory,  92 

Silver  haloids,  12 

Silver  sub-bromide  theory,  93 

Simplicity,  200 

Size  of  image,  149 

Sky,  202 

Sky-line,  201 

Slopes  of  characteristic  curve, 

43 

Soda,  234 

stock  formula  for,  236 
Sodium  hyposulphite,  5,  18 


INDEX 


Sodium  sulphide,  103,  132,  250 
Sodium  sulphide  solution,  for- 
mula for,  265 
Solid  solution  theory,  94 
Solio,  126 

Solution,  colloidal,  95 
Spectrum  photography,  253 
Speed  of  development,  221, 
222,  223 

of  plate,  221,  222,  223 
of  color  screen  plates,  223 
of  lenses,  167 
of  plates,  64 

of  plate  by  photometer,  287 
Spherical  aberration,  15 1 
Stain,  56,  1 18 

Staining  in  intensification,  loi 
Star,  12 

Starch  grains,  187 
Stock  pyro,  formula  for,  236 
Stock  soda,  formula  for,  236 
Stops,  61 

system  of  numbering,  61 
Strip  negative,  241 
Sub-bromide  theory,  93 
Subject,  character  of,  68 
Subjects,  classification  of,  70 
Subtraction  method,  180 
Sulphide  tones,  formula  for, 
265 

Sulphide  toning,  132,  264 
Supplies  per  student,  278 
System  of  numbering  stops,  63 
Systems  for  plate  speeds,  225 

Table  of  plate  speed  numbers, 
225 
Talbot,  6 


297 

Tank  development,  32,  57,  252 
Tannic  acid,  185,  191 
Telephoto  lens,  170 
Temperature  coefficient  of  de- 
velopers, 48,  50 
Test  for  correct  exposure,  239 
Thickness  of  film,  52 
Thin  negatives,  99 
Thiosulphate,  18 
Tilting  camera  effect,  213 
Tissue,  carbon,  137 
Todd,  F.  D.,  54 
Toning,  sulphide,  132 
Toning  P.O.P.,  126,  260 
Total  scale  of  papers,  129 
Transfer,  carbon  printing,  272 
Transmitted  light,  21 
Transmission,  21 
Transparencies,  121 
Traube  iodide  process,  181, 185 
Trimming,  216 
Tripack,  180 
Tungsten  lamp,  237 
Turnbull’s  blue,  124 

Underdeveloped  negative,  248 
Underdevelopment,  loi 
Underexposed  negative,  248 
Underexposure,  100 
period  of,  40 
Unequal  illumination,  163 
Unity,  199 
U.  S.  numbers,  63 

Virtual  focus,  146 
Vogel,  74 

Wall,  75 

Wallace,  82,  85,  254 


INDEX 


298 

Warner-Powrie,  191 
Watkins,  50,  64,  66,  71,  225 
Watkins  factor,  50 
Watkins  numbers,  66 
Watkins  speed  list,  220 
Wedgwood,  4 
Wellcome  exposure  record,  224 
Weston,  199 


Wet  collodion,  7,  139 
Wet  collodion  plates,  265 
Wide  angle  lens,  165 
Window  frame  picture,  209 
Wratten  and  Wainwright,  224 
Wynne,  225 

Zinc  etching,  139 


(6) 


3 3125  01130  4298 


